diff --git a/core/includes/theme.maintenance.inc b/core/includes/theme.maintenance.inc
index 80cc52cb81..310c9cd858 100644
--- a/core/includes/theme.maintenance.inc
+++ b/core/includes/theme.maintenance.inc
@@ -5,7 +5,6 @@
  * Theming for maintenance pages.
  */
 
-use Drupal\Component\Utility\Unicode;
 use Drupal\Core\Site\Settings;
 
 /**
@@ -29,7 +28,6 @@ function _drupal_maintenance_theme() {
   require_once __DIR__ . '/file.inc';
   require_once __DIR__ . '/module.inc';
   require_once __DIR__ . '/database.inc';
-  Unicode::check();
 
   // Install and update pages are treated differently to prevent theming overrides.
   if (defined('MAINTENANCE_MODE') && (MAINTENANCE_MODE == 'install' || MAINTENANCE_MODE == 'update')) {
diff --git a/core/lib/Drupal/Component/Utility/Unicode.php b/core/lib/Drupal/Component/Utility/Unicode.php
index e136a8971b..7cf2351603 100644
--- a/core/lib/Drupal/Component/Utility/Unicode.php
+++ b/core/lib/Drupal/Component/Utility/Unicode.php
@@ -87,13 +87,6 @@ class Unicode {
    */
   const STATUS_ERROR = -1;
 
-  /**
-   * Holds the multibyte capabilities of the current environment.
-   *
-   * @var int
-   */
-  protected static $status = 0;
-
   /**
    * Gets the current status of unicode/multibyte support on this environment.
    *
@@ -107,7 +100,13 @@ class Unicode {
    *     An error occurred. No unicode support.
    */
   public static function getStatus() {
-    return static::$status;
+    switch (static::check()) {
+      case 'mb_strlen':
+        return Unicode::STATUS_SINGLEBYTE;
+      case '':
+        return Unicode::STATUS_MULTIBYTE;
+    }
+    return Unicode::STATUS_ERROR;
   }
 
   /**
@@ -123,12 +122,16 @@ public static function getStatus() {
    *
    * @param int $status
    *   The new status of multibyte support.
+   *
+   * @deprecated in Drupal 8.6.0 and will be removed before Drupal 9.0.0. In
+   *   Drupal 9 there will be no way to set the status and in Drupal 8 this
+   *   ability has been removed because mb_*() functions are supplied using
+   *   Symfony's polyfill.
+   *
+   * @see https://www.drupal.org/node/2850048
    */
   public static function setStatus($status) {
-    if (!in_array($status, [static::STATUS_SINGLEBYTE, static::STATUS_MULTIBYTE, static::STATUS_ERROR])) {
-      throw new \InvalidArgumentException('Invalid status value for unicode support.');
-    }
-    static::$status = $status;
+    @trigger_error('\Drupal\Component\Utility\Unicode::setStatus() is deprecated in Drupal 8.6.0 and will be removed before Drupal 9.0.0. In Drupal 9 there will be no way to set the status and in Drupal 8 this ability has been removed because mb_*() functions are supplied using Symfony\'s polyfill. See https://www.drupal.org/node/2850048.', E_USER_DEPRECATED);
   }
 
   /**
@@ -143,38 +146,33 @@ public static function setStatus($status) {
    *   Otherwise, an empty string.
    */
   public static function check() {
+    // Set appropriate configuration.
+    mb_internal_encoding('utf-8');
+    mb_language('uni');
+
     // Check for mbstring extension.
-    if (!function_exists('mb_strlen')) {
-      static::$status = static::STATUS_SINGLEBYTE;
+    if (!extension_loaded('mbstring')) {
       return 'mb_strlen';
     }
 
     // Check mbstring configuration.
     if (ini_get('mbstring.func_overload') != 0) {
-      static::$status = static::STATUS_ERROR;
       return 'mbstring.func_overload';
     }
     if (ini_get('mbstring.encoding_translation') != 0) {
-      static::$status = static::STATUS_ERROR;
       return 'mbstring.encoding_translation';
     }
     // mbstring.http_input and mbstring.http_output are deprecated and empty by
     // default in PHP 5.6.
     if (version_compare(PHP_VERSION, '5.6.0') == -1) {
       if (ini_get('mbstring.http_input') != 'pass') {
-        static::$status = static::STATUS_ERROR;
         return 'mbstring.http_input';
       }
       if (ini_get('mbstring.http_output') != 'pass') {
-        static::$status = static::STATUS_ERROR;
         return 'mbstring.http_output';
       }
     }
 
-    // Set appropriate configuration.
-    mb_internal_encoding('utf-8');
-    mb_language('uni');
-    static::$status = static::STATUS_MULTIBYTE;
     return '';
   }
 
@@ -224,17 +222,7 @@ public static function encodingFromBOM($data) {
    *   Converted data or FALSE.
    */
   public static function convertToUtf8($data, $encoding) {
-    if (function_exists('iconv')) {
-      return @iconv($encoding, 'utf-8', $data);
-    }
-    elseif (function_exists('mb_convert_encoding')) {
-      return @mb_convert_encoding($data, 'utf-8', $encoding);
-    }
-    elseif (function_exists('recode_string')) {
-      return @recode_string($encoding . '..utf-8', $data);
-    }
-    // Cannot convert.
-    return FALSE;
+    return @iconv($encoding, 'utf-8', $data);
   }
 
   /**
@@ -281,15 +269,15 @@ public static function truncateBytes($string, $len) {
    *
    * @return int
    *   The length of the string.
+   *
+   * @deprecated in Drupal 8.6.0, will be removed before Drupal 9.0.0. Use
+   *   mb_strlen() instead.
+   *
+   * @see https://www.drupal.org/node/2850048
    */
   public static function strlen($text) {
-    if (static::getStatus() == static::STATUS_MULTIBYTE) {
-      return mb_strlen($text);
-    }
-    else {
-      // Do not count UTF-8 continuation bytes.
-      return strlen(preg_replace("/[\x80-\xBF]/", '', $text));
-    }
+    @trigger_error('\Drupal\Component\Utility\Unicode::strlen() is deprecated in Drupal 8.6.0 and will be removed before Drupal 9.0.0. Use mb_strlen() instead. See https://www.drupal.org/node/2850048.', E_USER_DEPRECATED);
+    return mb_strlen($text);
   }
 
   /**
@@ -300,18 +288,15 @@ public static function strlen($text) {
    *
    * @return string
    *   The string in uppercase.
+   *
+   * @deprecated in Drupal 8.6.0, will be removed before Drupal 9.0.0. Use
+   *   mb_strtoupper() instead.
+   *
+   * @see https://www.drupal.org/node/2850048
    */
   public static function strtoupper($text) {
-    if (static::getStatus() == static::STATUS_MULTIBYTE) {
-      return mb_strtoupper($text);
-    }
-    else {
-      // Use C-locale for ASCII-only uppercase.
-      $text = strtoupper($text);
-      // Case flip Latin-1 accented letters.
-      $text = preg_replace_callback('/\xC3[\xA0-\xB6\xB8-\xBE]/', '\Drupal\Component\Utility\Unicode::caseFlip', $text);
-      return $text;
-    }
+    @trigger_error('\Drupal\Component\Utility\Unicode::strtoupper() is deprecated in Drupal 8.6.0 and will be removed before Drupal 9.0.0. Use mb_strtoupper() instead. See https://www.drupal.org/node/2850048.', E_USER_DEPRECATED);
+    return mb_strtoupper($text);
   }
 
   /**
@@ -322,18 +307,15 @@ public static function strtoupper($text) {
    *
    * @return string
    *   The string in lowercase.
+   *
+   * @deprecated in Drupal 8.6.0, will be removed before Drupal 9.0.0. Use
+   *   mb_strtolower() instead.
+   *
+   * @see https://www.drupal.org/node/2850048
    */
   public static function strtolower($text) {
-    if (static::getStatus() == static::STATUS_MULTIBYTE) {
-      return mb_strtolower($text);
-    }
-    else {
-      // Use C-locale for ASCII-only lowercase.
-      $text = strtolower($text);
-      // Case flip Latin-1 accented letters.
-      $text = preg_replace_callback('/\xC3[\x80-\x96\x98-\x9E]/', '\Drupal\Component\Utility\Unicode::caseFlip', $text);
-      return $text;
-    }
+    @trigger_error('\Drupal\Component\Utility\Unicode::strtolower() is deprecated in Drupal 8.6.0 and will be removed before Drupal 9.0.0. Use mb_strtolower() instead. See https://www.drupal.org/node/2850048.', E_USER_DEPRECATED);
+    return mb_strtolower($text);
   }
 
   /**
@@ -346,7 +328,7 @@ public static function strtolower($text) {
    *   The string with the first character as uppercase.
    */
   public static function ucfirst($text) {
-    return static::strtoupper(static::substr($text, 0, 1)) . static::substr($text, 1);
+    return mb_strtoupper(mb_substr($text, 0, 1)) . mb_substr($text, 1);
   }
 
   /**
@@ -362,7 +344,7 @@ public static function ucfirst($text) {
    */
   public static function lcfirst($text) {
     // Note: no mbstring equivalent!
-    return static::strtolower(static::substr($text, 0, 1)) . static::substr($text, 1);
+    return mb_strtolower(mb_substr($text, 0, 1)) . mb_substr($text, 1);
   }
 
   /**
@@ -379,7 +361,7 @@ public static function lcfirst($text) {
   public static function ucwords($text) {
     $regex = '/(^|[' . static::PREG_CLASS_WORD_BOUNDARY . '])([^' . static::PREG_CLASS_WORD_BOUNDARY . '])/u';
     return preg_replace_callback($regex, function (array $matches) {
-      return $matches[1] . Unicode::strtoupper($matches[2]);
+      return $matches[1] . mb_strtoupper($matches[2]);
     }, $text);
   }
 
@@ -399,92 +381,15 @@ public static function ucwords($text) {
    *
    * @return string
    *   The shortened string.
+   *
+   * @deprecated in Drupal 8.6.0, will be removed before Drupal 9.0.0. Use
+   *   mb_substr() instead.
+   *
+   * @see https://www.drupal.org/node/2850048
    */
   public static function substr($text, $start, $length = NULL) {
-    if (static::getStatus() == static::STATUS_MULTIBYTE) {
-      return $length === NULL ? mb_substr($text, $start) : mb_substr($text, $start, $length);
-    }
-    else {
-      $strlen = strlen($text);
-      // Find the starting byte offset.
-      $bytes = 0;
-      if ($start > 0) {
-        // Count all the characters except continuation bytes from the start
-        // until we have found $start characters or the end of the string.
-        $bytes = -1; $chars = -1;
-        while ($bytes < $strlen - 1 && $chars < $start) {
-          $bytes++;
-          $c = ord($text[$bytes]);
-          if ($c < 0x80 || $c >= 0xC0) {
-            $chars++;
-          }
-        }
-      }
-      elseif ($start < 0) {
-        // Count all the characters except continuation bytes from the end
-        // until we have found abs($start) characters.
-        $start = abs($start);
-        $bytes = $strlen; $chars = 0;
-        while ($bytes > 0 && $chars < $start) {
-          $bytes--;
-          $c = ord($text[$bytes]);
-          if ($c < 0x80 || $c >= 0xC0) {
-            $chars++;
-          }
-        }
-      }
-      $istart = $bytes;
-
-      // Find the ending byte offset.
-      if ($length === NULL) {
-        $iend = $strlen;
-      }
-      elseif ($length > 0) {
-        // Count all the characters except continuation bytes from the starting
-        // index until we have found $length characters or reached the end of
-        // the string, then backtrace one byte.
-        $iend = $istart - 1;
-        $chars = -1;
-        $last_real = FALSE;
-        while ($iend < $strlen - 1 && $chars < $length) {
-          $iend++;
-          $c = ord($text[$iend]);
-          $last_real = FALSE;
-          if ($c < 0x80 || $c >= 0xC0) {
-            $chars++;
-            $last_real = TRUE;
-          }
-        }
-        // Backtrace one byte if the last character we found was a real
-        // character and we don't need it.
-        if ($last_real && $chars >= $length) {
-          $iend--;
-        }
-      }
-      elseif ($length < 0) {
-        // Count all the characters except continuation bytes from the end
-        // until we have found abs($start) characters, then backtrace one byte.
-        $length = abs($length);
-        $iend = $strlen; $chars = 0;
-        while ($iend > 0 && $chars < $length) {
-          $iend--;
-          $c = ord($text[$iend]);
-          if ($c < 0x80 || $c >= 0xC0) {
-            $chars++;
-          }
-        }
-        // Backtrace one byte if we are not at the beginning of the string.
-        if ($iend > 0) {
-          $iend--;
-        }
-      }
-      else {
-        // $length == 0, return an empty string.
-        return '';
-      }
-
-      return substr($text, $istart, max(0, $iend - $istart + 1));
-    }
+    @trigger_error('\Drupal\Component\Utility\Unicode::substr() is deprecated in Drupal 8.6.0 and will be removed before Drupal 9.0.0. Use mb_substr() instead. See https://www.drupal.org/node/2850048.', E_USER_DEPRECATED);
+    return mb_substr($text, $start, $length);
   }
 
   /**
@@ -526,15 +431,15 @@ public static function truncate($string, $max_length, $wordsafe = FALSE, $add_el
     $max_length = max($max_length, 0);
     $min_wordsafe_length = max($min_wordsafe_length, 0);
 
-    if (static::strlen($string) <= $max_length) {
+    if (mb_strlen($string) <= $max_length) {
       // No truncation needed, so don't add ellipsis, just return.
       return $string;
     }
 
     if ($add_ellipsis) {
       // Truncate ellipsis in case $max_length is small.
-      $ellipsis = static::substr('…', 0, $max_length);
-      $max_length -= static::strlen($ellipsis);
+      $ellipsis = mb_substr('…', 0, $max_length);
+      $max_length -= mb_strlen($ellipsis);
       $max_length = max($max_length, 0);
     }
 
@@ -553,11 +458,11 @@ public static function truncate($string, $max_length, $wordsafe = FALSE, $add_el
         $string = $matches[1];
       }
       else {
-        $string = static::substr($string, 0, $max_length);
+        $string = mb_substr($string, 0, $max_length);
       }
     }
     else {
-      $string = static::substr($string, 0, $max_length);
+      $string = mb_substr($string, 0, $max_length);
     }
 
     if ($add_ellipsis) {
@@ -583,7 +488,7 @@ public static function truncate($string, $max_length, $wordsafe = FALSE, $add_el
    *   $str2, and 0 if they are equal.
    */
   public static function strcasecmp($str1, $str2) {
-    return strcmp(static::strtoupper($str1), static::strtoupper($str2));
+    return strcmp(mb_strtoupper($str1), mb_strtoupper($str2));
   }
 
   /**
@@ -715,18 +620,15 @@ public static function validateUtf8($text) {
    *   The position where $needle occurs in $haystack, always relative to the
    *   beginning (independent of $offset), or FALSE if not found. Note that
    *   a return value of 0 is not the same as FALSE.
+   *
+   * @deprecated in Drupal 8.6.0, will be removed before Drupal 9.0.0. Use
+   *   mb_strpos() instead.
+   *
+   * @see https://www.drupal.org/node/2850048
    */
   public static function strpos($haystack, $needle, $offset = 0) {
-    if (static::getStatus() == static::STATUS_MULTIBYTE) {
-      return mb_strpos($haystack, $needle, $offset);
-    }
-    else {
-      // Remove Unicode continuation characters, to be compatible with
-      // Unicode::strlen() and Unicode::substr().
-      $haystack = preg_replace("/[\x80-\xBF]/", '', $haystack);
-      $needle = preg_replace("/[\x80-\xBF]/", '', $needle);
-      return strpos($haystack, $needle, $offset);
-    }
+    @trigger_error('\Drupal\Component\Utility\Unicode::strpos() is deprecated in Drupal 8.6.0 and will be removed before Drupal 9.0.0. Use mb_strpos() instead. See https://www.drupal.org/node/2850048.', E_USER_DEPRECATED);
+    return mb_strpos($haystack, $needle, $offset);
   }
 
 }
diff --git a/core/lib/Drupal/Component/Utility/composer.json b/core/lib/Drupal/Component/Utility/composer.json
index 3b8b5de5b8..b89e813903 100644
--- a/core/lib/Drupal/Component/Utility/composer.json
+++ b/core/lib/Drupal/Component/Utility/composer.json
@@ -7,7 +7,9 @@
   "require": {
     "php": ">=5.5.9",
     "paragonie/random_compat": "^1.0|^2.0",
-    "drupal/core-render": "^8.2"
+    "drupal/core-render": "^8.2",
+    "symfony/polyfill-iconv": "~1.0",
+    "symfony/polyfill-mbstring": "~1.0"
   },
   "autoload": {
     "psr-4": {
diff --git a/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
index 37ed0e97a6..55f446e2b7 100644
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -5,7 +5,6 @@
 use Composer\Autoload\ClassLoader;
 use Drupal\Component\Assertion\Handle;
 use Drupal\Component\FileCache\FileCacheFactory;
-use Drupal\Component\Utility\Unicode;
 use Drupal\Component\Utility\UrlHelper;
 use Drupal\Core\Cache\DatabaseBackend;
 use Drupal\Core\Config\BootstrapConfigStorageFactory;
@@ -989,8 +988,9 @@ public static function bootEnvironment($app_root = NULL) {
     // numbers handling.
     setlocale(LC_ALL, 'C');
 
-    // Detect string handling method.
-    Unicode::check();
+    // Set appropriate configuration for multi-byte strings.
+    mb_internal_encoding('utf-8');
+    mb_language('uni');
 
     // Indicate that code is operating in a test child site.
     if (!defined('DRUPAL_TEST_IN_CHILD_SITE')) {
diff --git a/core/modules/image/src/Plugin/ImageEffect/ConvertImageEffect.php b/core/modules/image/src/Plugin/ImageEffect/ConvertImageEffect.php
index 28d51792c8..cb7a067847 100644
--- a/core/modules/image/src/Plugin/ImageEffect/ConvertImageEffect.php
+++ b/core/modules/image/src/Plugin/ImageEffect/ConvertImageEffect.php
@@ -64,7 +64,7 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
     $extensions = \Drupal::service('image.toolkit.manager')->getDefaultToolkit()->getSupportedExtensions();
     $options = array_combine(
       $extensions,
-      array_map(['\Drupal\Component\Utility\Unicode', 'strtoupper'], $extensions)
+      array_map('mb_strtoupper', $extensions)
     );
     $form['extension'] = [
       '#type' => 'select',
diff --git a/core/modules/migrate/tests/src/Unit/process/CallbackTest.php b/core/modules/migrate/tests/src/Unit/process/CallbackTest.php
index 91eb2e7603..97a2d0316e 100644
--- a/core/modules/migrate/tests/src/Unit/process/CallbackTest.php
+++ b/core/modules/migrate/tests/src/Unit/process/CallbackTest.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\Tests\migrate\Unit\process;
 
+use Drupal\Component\Utility\Unicode;
 use Drupal\migrate\Plugin\migrate\process\Callback;
 
 /**
@@ -37,9 +38,9 @@ public function testCallbackWithFunction() {
    * Test callback with a class method as callable.
    */
   public function testCallbackWithClassMethod() {
-    $this->plugin->setCallable(['\Drupal\Component\Utility\Unicode', 'strtolower']);
-    $value = $this->plugin->transform('FooBar', $this->migrateExecutable, $this->row, 'destinationproperty');
-    $this->assertSame('foobar', $value);
+    $this->plugin->setCallable([Unicode::class, 'ucfirst']);
+    $value = $this->plugin->transform('foobar', $this->migrateExecutable, $this->row, 'destinationproperty');
+    $this->assertSame('Foobar', $value);
   }
 
 }
diff --git a/core/modules/node/tests/src/Kernel/SummaryLengthTest.php b/core/modules/node/tests/src/Kernel/SummaryLengthTest.php
index fa15c0e6ab..eecb0d0188 100644
--- a/core/modules/node/tests/src/Kernel/SummaryLengthTest.php
+++ b/core/modules/node/tests/src/Kernel/SummaryLengthTest.php
@@ -2,7 +2,6 @@
 
 namespace Drupal\Tests\node\Kernel;
 
-use Drupal\Component\Utility\Unicode;
 use Drupal\Core\Datetime\Entity\DateFormat;
 use Drupal\KernelTests\KernelTestBase;
 use Drupal\node\Entity\Node;
@@ -72,9 +71,6 @@ protected function setUp() {
       'label' => 'Fallback',
       'pattern' => 'Y-m-d',
     ])->save();
-
-    // Enable multibyte support.
-    Unicode::setStatus(Unicode::STATUS_MULTIBYTE);
   }
 
   /**
diff --git a/core/modules/search/search.module b/core/modules/search/search.module
index 5be9206bc3..3980243c92 100644
--- a/core/modules/search/search.module
+++ b/core/modules/search/search.module
@@ -846,7 +846,7 @@ function _search_find_match_with_simplify($key, $text, $boundary, $langcode = NU
   $new_key = Unicode::strtolower($temp);
   $new_key = \Drupal::service('transliteration')->removeDiacritics($new_key);
   if (preg_match('/' . $preceded_by_boundary . preg_quote($new_key, '/') . $followed_by_boundary . '/u', ' ' . $new_text . ' ')) {
-    $position = Unicode::strpos($new_text, $new_key);
+    $position = mb_strpos($new_text, $new_key);
     return Unicode::substr($text, $position, Unicode::strlen($new_key));
   }
 
diff --git a/core/tests/Drupal/Tests/Component/Utility/UnicodeTest.php b/core/tests/Drupal/Tests/Component/Utility/UnicodeTest.php
index 960d1812ad..3dc731ce96 100644
--- a/core/tests/Drupal/Tests/Component/Utility/UnicodeTest.php
+++ b/core/tests/Drupal/Tests/Component/Utility/UnicodeTest.php
@@ -15,56 +15,11 @@
 class UnicodeTest extends TestCase {
 
   /**
-   * {@inheritdoc}
-   *
-   * @covers ::check
-   */
-  protected function setUp() {
-    // Initialize unicode component.
-    Unicode::check();
-  }
-
-  /**
-   * Getting and settings the multibyte environment status.
-   *
-   * @dataProvider providerTestStatus
-   * @covers ::getStatus
-   * @covers ::setStatus
-   */
-  public function testStatus($value, $expected, $invalid = FALSE) {
-    if ($invalid) {
-      if (method_exists($this, 'expectException')) {
-        $this->expectException('InvalidArgumentException');
-      }
-      else {
-        $this->setExpectedException('InvalidArgumentException');
-      }
-    }
-    Unicode::setStatus($value);
-    $this->assertEquals($expected, Unicode::getStatus());
-  }
-
-  /**
-   * Data provider for testStatus().
-   *
-   * @see testStatus()
-   *
-   * @return array
-   *   An array containing:
-   *     - The status value to set.
-   *     - The status value to expect after setting the new value.
-   *     - (optional) Boolean indicating invalid status. Defaults to FALSE.
+   * @group legacy
+   * @expectedDeprecation \Drupal\Component\Utility\Unicode::setStatus() is deprecated in Drupal 8.6.0 and will be removed before Drupal 9.0.0. In Drupal 9 there will be no way to set the status and in Drupal 8 this ability has been removed because mb_*() functions are supplied using Symfony's polyfill. See https://www.drupal.org/node/2850048.
    */
-  public function providerTestStatus() {
-    return [
-      [Unicode::STATUS_SINGLEBYTE, Unicode::STATUS_SINGLEBYTE],
-      [rand(10, 100), Unicode::STATUS_SINGLEBYTE, TRUE],
-      [rand(10, 100), Unicode::STATUS_SINGLEBYTE, TRUE],
-      [Unicode::STATUS_MULTIBYTE, Unicode::STATUS_MULTIBYTE],
-      [rand(10, 100), Unicode::STATUS_MULTIBYTE, TRUE],
-      [Unicode::STATUS_ERROR, Unicode::STATUS_ERROR],
-      [Unicode::STATUS_MULTIBYTE, Unicode::STATUS_MULTIBYTE],
-    ];
+  public function testSetStatus() {
+    Unicode::setStatus(Unicode::STATUS_SINGLEBYTE);
   }
 
   /**
@@ -101,10 +56,10 @@ public function providerTestMimeHeader() {
    * @dataProvider providerStrtolower
    * @covers ::strtolower
    * @covers ::caseFlip
+   * @group legacy
+   * @expectedDeprecation \Drupal\Component\Utility\Unicode::strtolower() is deprecated in Drupal 8.6.0 and will be removed before Drupal 9.0.0. Use mb_strtolower() instead. See https://www.drupal.org/node/2850048.
    */
-  public function testStrtolower($text, $expected, $multibyte = FALSE) {
-    $status = $multibyte ? Unicode::STATUS_MULTIBYTE : Unicode::STATUS_SINGLEBYTE;
-    Unicode::setStatus($status);
+  public function testStrtolower($text, $expected) {
     $this->assertEquals($expected, Unicode::strtolower($text));
   }
 
@@ -114,22 +69,14 @@ public function testStrtolower($text, $expected, $multibyte = FALSE) {
    * @see testStrtolower()
    *
    * @return array
-   *   An array containing a string, its lowercase version and whether it should
-   *   be processed as multibyte.
+   *   An array containing a string and its lowercase version.
    */
   public function providerStrtolower() {
-    $cases = [
+    return [
       ['tHe QUIcK bRoWn', 'the quick brown'],
       ['FrançAIS is ÜBER-åwesome', 'français is über-åwesome'],
+      ['ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΣὨ', 'αβγδεζηθικλμνξοσὠ'],
     ];
-    foreach ($cases as $case) {
-      // Test the same string both in multibyte and singlebyte conditions.
-      array_push($case, TRUE);
-      $cases[] = $case;
-    }
-    // Add a multibyte string.
-    $cases[] = ['ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΣὨ', 'αβγδεζηθικλμνξοσὠ', TRUE];
-    return $cases;
   }
 
   /**
@@ -138,10 +85,10 @@ public function providerStrtolower() {
    * @dataProvider providerStrtoupper
    * @covers ::strtoupper
    * @covers ::caseFlip
+   * @group legacy
+   * @expectedDeprecation \Drupal\Component\Utility\Unicode::strtoupper() is deprecated in Drupal 8.6.0 and will be removed before Drupal 9.0.0. Use mb_strtoupper() instead. See https://www.drupal.org/node/2850048.
    */
-  public function testStrtoupper($text, $expected, $multibyte = FALSE) {
-    $status = $multibyte ? Unicode::STATUS_MULTIBYTE : Unicode::STATUS_SINGLEBYTE;
-    Unicode::setStatus($status);
+  public function testStrtoupper($text, $expected) {
     $this->assertEquals($expected, Unicode::strtoupper($text));
   }
 
@@ -151,22 +98,14 @@ public function testStrtoupper($text, $expected, $multibyte = FALSE) {
    * @see testStrtoupper()
    *
    * @return array
-   *   An array containing a string, its uppercase version and whether it should
-   *   be processed as multibyte.
+   *   An array containing a string and its uppercase version.
    */
   public function providerStrtoupper() {
-    $cases = [
+    return [
       ['tHe QUIcK bRoWn', 'THE QUICK BROWN'],
       ['FrançAIS is ÜBER-åwesome', 'FRANÇAIS IS ÜBER-ÅWESOME'],
+      ['αβγδεζηθικλμνξοσὠ', 'ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΣὨ'],
     ];
-    foreach ($cases as $case) {
-      // Test the same string both in multibyte and singlebyte conditions.
-      array_push($case, TRUE);
-      $cases[] = $case;
-    }
-    // Add a multibyte string.
-    $cases[] = ['αβγδεζηθικλμνξοσὠ', 'ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΣὨ', TRUE];
-    return $cases;
   }
 
   /**
@@ -204,9 +143,7 @@ public function providerUcfirst() {
    * @dataProvider providerLcfirst
    * @covers ::lcfirst
    */
-  public function testLcfirst($text, $expected, $multibyte = FALSE) {
-    $status = $multibyte ? Unicode::STATUS_MULTIBYTE : Unicode::STATUS_SINGLEBYTE;
-    Unicode::setStatus($status);
+  public function testLcfirst($text, $expected) {
     $this->assertEquals($expected, Unicode::lcfirst($text));
   }
 
@@ -216,8 +153,7 @@ public function testLcfirst($text, $expected, $multibyte = FALSE) {
    * @see testLcfirst()
    *
    * @return array
-   *   An array containing a string, its lowercase version and whether it should
-   *   be processed as multibyte.
+   *   An array containing a string and its lowercase version.
    */
   public function providerLcfirst() {
     return [
@@ -226,7 +162,7 @@ public function providerLcfirst() {
       ['Über', 'über'],
       ['Åwesome', 'åwesome'],
       // Add a multibyte string.
-      ['ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΣὨ', 'αΒΓΔΕΖΗΘΙΚΛΜΝΞΟΣὨ', TRUE],
+      ['ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΣὨ', 'αΒΓΔΕΖΗΘΙΚΛΜΝΞΟΣὨ'],
     ];
   }
 
@@ -236,9 +172,7 @@ public function providerLcfirst() {
    * @dataProvider providerUcwords
    * @covers ::ucwords
    */
-  public function testUcwords($text, $expected, $multibyte = FALSE) {
-    $status = $multibyte ? Unicode::STATUS_MULTIBYTE : Unicode::STATUS_SINGLEBYTE;
-    Unicode::setStatus($status);
+  public function testUcwords($text, $expected) {
     $this->assertEquals($expected, Unicode::ucwords($text));
   }
 
@@ -248,8 +182,7 @@ public function testUcwords($text, $expected, $multibyte = FALSE) {
    * @see testUcwords()
    *
    * @return array
-   *   An array containing a string, its capitalized version and whether it should
-   *   be processed as multibyte.
+   *   An array containing a string and its capitalized version.
    */
   public function providerUcwords() {
     return [
@@ -260,7 +193,7 @@ public function providerUcwords() {
       // Make sure we don't mangle extra spaces.
       ['frànçAIS is  über-åwesome', 'FrànçAIS Is  Über-Åwesome'],
       // Add a multibyte string.
-      ['σion', 'Σion', TRUE],
+      ['σion', 'Σion'],
     ];
   }
 
@@ -269,13 +202,10 @@ public function providerUcwords() {
    *
    * @dataProvider providerStrlen
    * @covers ::strlen
+   * @group legacy
+   * @expectedDeprecation \Drupal\Component\Utility\Unicode::strlen() is deprecated in Drupal 8.6.0 and will be removed before Drupal 9.0.0. Use mb_strlen() instead. See https://www.drupal.org/node/2850048.
    */
   public function testStrlen($text, $expected) {
-    // Run through multibyte code path.
-    Unicode::setStatus(Unicode::STATUS_MULTIBYTE);
-    $this->assertEquals($expected, Unicode::strlen($text));
-    // Run through singlebyte code path.
-    Unicode::setStatus(Unicode::STATUS_SINGLEBYTE);
     $this->assertEquals($expected, Unicode::strlen($text));
   }
 
@@ -300,13 +230,10 @@ public function providerStrlen() {
    *
    * @dataProvider providerSubstr
    * @covers ::substr
+   * @group legacy
+   * @expectedDeprecation \Drupal\Component\Utility\Unicode::substr() is deprecated in Drupal 8.6.0 and will be removed before Drupal 9.0.0. Use mb_substr() instead. See https://www.drupal.org/node/2850048.
    */
   public function testSubstr($text, $start, $length, $expected) {
-    // Run through multibyte code path.
-    Unicode::setStatus(Unicode::STATUS_MULTIBYTE);
-    $this->assertEquals($expected, Unicode::substr($text, $start, $length));
-    // Run through singlebyte code path.
-    Unicode::setStatus(Unicode::STATUS_SINGLEBYTE);
     $this->assertEquals($expected, Unicode::substr($text, $start, $length));
   }
 
@@ -553,13 +480,10 @@ public function providerTestConvertToUtf8() {
    *
    * @dataProvider providerStrpos
    * @covers ::strpos
+   * @group legacy
+   * @expectedDeprecation \Drupal\Component\Utility\Unicode::strpos() is deprecated in Drupal 8.6.0 and will be removed before Drupal 9.0.0. Use mb_strpos() instead. See https://www.drupal.org/node/2850048.
    */
   public function testStrpos($haystack, $needle, $offset, $expected) {
-    // Run through multibyte code path.
-    Unicode::setStatus(Unicode::STATUS_MULTIBYTE);
-    $this->assertEquals($expected, Unicode::strpos($haystack, $needle, $offset));
-    // Run through singlebyte code path.
-    Unicode::setStatus(Unicode::STATUS_SINGLEBYTE);
     $this->assertEquals($expected, Unicode::strpos($haystack, $needle, $offset));
   }
 
diff --git a/core/tests/bootstrap.php b/core/tests/bootstrap.php
index 7eb6ecb70c..f46d1e8f11 100644
--- a/core/tests/bootstrap.php
+++ b/core/tests/bootstrap.php
@@ -169,6 +169,10 @@ function drupal_phpunit_populate_class_loader() {
 // @see \Drupal\Core\DrupalKernel::bootEnvironment()
 setlocale(LC_ALL, 'C');
 
+// Set appropriate configuration for multi-byte strings.
+mb_internal_encoding('utf-8');
+mb_language('uni');
+
 // Set the default timezone. While this doesn't cause any tests to fail, PHP
 // complains if 'date.timezone' is not set in php.ini. The Australia/Sydney
 // timezone is chosen so all tests are run using an edge case scenario (UTC+10