diff --git a/core/lib/Drupal/Core/PageCache/RequestPolicyInterface.php b/core/lib/Drupal/Core/PageCache/RequestPolicyInterface.php
index b287e60..3a4ef21 100644
--- a/core/lib/Drupal/Core/PageCache/RequestPolicyInterface.php
+++ b/core/lib/Drupal/Core/PageCache/RequestPolicyInterface.php
@@ -13,7 +13,7 @@
  * Defines the interface for request policy implementations.
  *
  * The request policy is evaluated in order to determine whether delivery of a
- * cached page should be attempted. Calling should do so if static::ALLOW is
+ * cached page should be attempted. The caller should do so if static::ALLOW is
  * returned from the check() method.
  */
 interface RequestPolicyInterface {
@@ -31,6 +31,16 @@
   /**
    * Determines whether delivery of a cached page should be attempted.
    *
+   * Note that the request-policy check runs very early. In particular it is
+   * not possible to determine the logged in user. Also the current route match
+   * is not yet present when the check runs. Therefore, request-policy checks
+   * need to be designed in a way such that they do not depend on any other
+   * service and only take in account the information present on the incoming
+   * request.
+   *
+   * When matching against the request path, special attention is needed to
+   * support path prefixes which are often used on multilingual sites.
+   *
    * @param \Symfony\Component\HttpFoundation\Request $request
    *   The incoming request object.
    *
diff --git a/core/modules/toolbar/src/PageCache/AllowToolbarPath.php b/core/modules/toolbar/src/PageCache/AllowToolbarPath.php
index 47a4989..81d5734 100644
--- a/core/modules/toolbar/src/PageCache/AllowToolbarPath.php
+++ b/core/modules/toolbar/src/PageCache/AllowToolbarPath.php
@@ -22,15 +22,10 @@ class AllowToolbarPath implements RequestPolicyInterface {
    * {@inheritdoc}
    */
   public function check(Request $request) {
-    $path = $request->getPathInfo();
-    $toolbar_path_offset = strpos($path, '/toolbar/subtrees/');
-    if ($toolbar_path_offset !== FALSE) {
-      // Also accept paths matching at the first slash in order to cater for
-      // sites using language prefixes.
-      $second_slash = strpos($path, '/', 1);
-      if ($toolbar_path_offset === 0 || $toolbar_path_offset === $second_slash) {
-        return static::ALLOW;
-      }
+    // Note that this regular expression matches the end of pathinfo in order to
+    // support multilingual sites using path prefixes.
+    if (preg_match('#/toolbar/subtrees/[^/]+(/[^/]+)?$#', $request->getPathInfo())) {
+      return static::ALLOW;
     }
   }
 
diff --git a/core/modules/toolbar/tests/src/Unit/PageCache/AllowToolbarPathTest.php b/core/modules/toolbar/tests/src/Unit/PageCache/AllowToolbarPathTest.php
index a783c71..aef4ec8 100644
--- a/core/modules/toolbar/tests/src/Unit/PageCache/AllowToolbarPathTest.php
+++ b/core/modules/toolbar/tests/src/Unit/PageCache/AllowToolbarPathTest.php
@@ -51,9 +51,13 @@ public function providerTestAllowToolbarPath() {
     return [
       [NULL, '/'],
       [NULL, '/other-path?q=/toolbar/subtrees/'],
+      [NULL, '/toolbar/subtrees/'],
+      [NULL, '/toolbar/subtrees/some-hash/langcode/additional-stuff'],
       [RequestPolicyInterface::ALLOW, '/de/toolbar/subtrees/abcd'],
-      [RequestPolicyInterface::ALLOW, '/en-us/toolbar/subtrees/'],
+      [RequestPolicyInterface::ALLOW, '/en-us/toolbar/subtrees/xyz'],
+      [RequestPolicyInterface::ALLOW, '/en-us/toolbar/subtrees/xyz/de'],
       [RequestPolicyInterface::ALLOW, '/toolbar/subtrees/some-hash'],
+      [RequestPolicyInterface::ALLOW, '/toolbar/subtrees/some-hash/en'],
     ];
   }