diff -u b/core/lib/Drupal/Core/StringTranslation/TranslationInterface.php b/core/lib/Drupal/Core/StringTranslation/TranslationInterface.php
--- b/core/lib/Drupal/Core/StringTranslation/TranslationInterface.php
+++ b/core/lib/Drupal/Core/StringTranslation/TranslationInterface.php
@@ -104,14 +104,2 @@
 
-   /**
-    * Returns the number of plurals supported by a given language.
-    *
-    * @param null|string $langcode
-    *   (optional) The language code. If not provided, the current language
-    *   will be used.
-    *
-    * @return int
-    *   Number of plural variants supported by the given language.
-    */
-   public function getNumberOfPlurals($langcode = NULL);
-
 }
@@ -137,2 +125,14 @@
 
+   /**
+    * Returns the number of plurals supported by a given language.
+    *
+    * @param null|string $langcode
+    *   (optional) The language code. If not provided, the current language
+    *   will be used.
+    *
+    * @return int
+    *   Number of plural variants supported by the given language.
+    */
+   public function getNumberOfPlurals($langcode = NULL);
+
 }
diff -u b/core/modules/locale/src/PluralFormula.php b/core/modules/locale/src/PluralFormula.php
--- b/core/modules/locale/src/PluralFormula.php
+++ b/core/modules/locale/src/PluralFormula.php
@@ -65,7 +65,7 @@
       'plurals' => $plural_count,
       'formula' => $formula,
     ];
-    $this->state->set('locale.translation.plurals', $this->formulae);
+    $this->state->set('locale.translation.formulae', $this->formulae);
     return $this;
   }
 
@@ -76,7 +76,7 @@
     // Ensure that the formulae are loaded.
     $this->loadFormulae();
 
-    // We also 2 plurals if there is no explicit information yet.
+    // Set the langcode to use.
     $langcode = $langcode ?: $this->languageManager->getCurrentLanguage()->getId();
 
     // We assume 2 plurals if there is no explicit information yet.
@@ -95,13 +95,13 @@
   }
 
   /**
-   * Loads the formulae and stores them on the object if not set.
+   * Loads the formulae and stores them on the PluralFormula object if not set.
    *
    * @return []
    */
   protected function loadFormulae() {
     if (!isset($this->formulae)) {
-      $this->formulae = $this->state->get('locale.translation.plurals', []);
+      $this->formulae = $this->state->get('locale.translation.formulae', []);
     }
   }
 
diff -u b/core/modules/locale/src/PluralFormulaInterface.php b/core/modules/locale/src/PluralFormulaInterface.php
--- b/core/modules/locale/src/PluralFormulaInterface.php
+++ b/core/modules/locale/src/PluralFormulaInterface.php
@@ -18,7 +18,7 @@
    * @param int $plural_count
    *   The number of plural forms.
    * @param array $formula
-   *   An array for formulas.
+   *   An array of formulae.
    *
    * @return self
    *   The PluralFormula object.
@@ -44,7 +44,7 @@
    *   The language code to get the formula for.
    *
    * @return array
-   *   An array for formulas.
+   *   An array of formulae.
    */
   public function getFormula($langcode);
 
diff -u b/core/tests/Drupal/KernelTests/Core/StringTranslation/TranslationStringTest.php b/core/tests/Drupal/KernelTests/Core/StringTranslation/TranslationStringTest.php
--- b/core/tests/Drupal/KernelTests/Core/StringTranslation/TranslationStringTest.php
+++ b/core/tests/Drupal/KernelTests/Core/StringTranslation/TranslationStringTest.php
@@ -60,7 +60,7 @@
    * Reboots the kernel to set custom translations in Settings.
    */
   protected function rebootAndPrepareSettings() {
-    // Reboot the container so that difference services are injected and the new
+    // Reboot the container so that different services are injected and the new
     // settings are picked.
     $kernel = $this->container->get('kernel');
     $kernel->shutdown();