diff --git a/core/lib/Drupal/Core/Language/LanguageInterface.php b/core/lib/Drupal/Core/Language/LanguageInterface.php
index 2568cb33f303..9e9907a0cc8a 100644
--- a/core/lib/Drupal/Core/Language/LanguageInterface.php
+++ b/core/lib/Drupal/Core/Language/LanguageInterface.php
@@ -51,6 +51,13 @@ interface LanguageInterface {
    */
   const LANGCODE_SITE_DEFAULT = 'site_default';
 
+  /**
+   * A regex for validating language codes according to W3C specifications.
+   *
+   * @see https://www.w3.org/International/articles/language-tags/
+   */
+  const VALID_LANGCODE_REGEX = '[a-zA-Z]{1,8}(-[a-zA-Z0-9]{1,8})*';
+
   /**
    * The language state when referring to configurable languages.
    */
diff --git a/core/lib/Drupal/Core/StringTranslation/Translator/FileTranslation.php b/core/lib/Drupal/Core/StringTranslation/Translator/FileTranslation.php
index 327b763b184a..b4c67ed80e2a 100644
--- a/core/lib/Drupal/Core/StringTranslation/Translator/FileTranslation.php
+++ b/core/lib/Drupal/Core/StringTranslation/Translator/FileTranslation.php
@@ -5,6 +5,7 @@ namespace Drupal\Core\StringTranslation\Translator;
 use Drupal\Component\Gettext\PoStreamReader;
 use Drupal\Component\Gettext\PoMemoryWriter;
 use Drupal\Core\File\FileSystemInterface;
+use Drupal\Core\Language\LanguageInterface;
 
 /**
  * File based string translation.
@@ -102,7 +103,7 @@ class FileTranslation extends StaticTranslation {
     // The file name matches: drupal-[release version].[language code].po
     // When provided the $langcode is use as language code. If not provided all
     // language codes will match.
-    return '!drupal-[0-9a-z\.-]+\.' . (!empty($langcode) ? preg_quote($langcode, '!') : '[^\.]+') . '\.po$!';
+    return '!drupal-[0-9]+\.[0-9]+\.([0-9]+|x)(-[a-z]+[0-9]*)?\.' . (!empty($langcode) ? preg_quote($langcode, '!') : LanguageInterface::VALID_LANGCODE_REGEX) . '\.po$!';
   }
 
   /**
diff --git a/core/modules/language/src/Form/LanguageFormBase.php b/core/modules/language/src/Form/LanguageFormBase.php
index d30c4b692285..2c9663574788 100644
--- a/core/modules/language/src/Form/LanguageFormBase.php
+++ b/core/modules/language/src/Form/LanguageFormBase.php
@@ -95,7 +95,7 @@ abstract class LanguageFormBase extends EntityForm {
    */
   public function validateCommon(array $form, FormStateInterface $form_state) {
     // Ensure sane field values for langcode and name.
-    if (!isset($form['langcode_view']) && !preg_match('@^[a-zA-Z]{1,8}(-[a-zA-Z0-9]{1,8})*$@', $form_state->getValue('langcode'))) {
+    if (!isset($form['langcode_view']) && !preg_match('@^' . LanguageInterface::VALID_LANGCODE_REGEX . '$@', $form_state->getValue('langcode'))) {
       $form_state->setErrorByName('langcode', $this->t('%field must be a valid language tag as <a href=":url">defined by the W3C</a>.', [
         '%field' => $form['langcode']['#title'],
         ':url' => 'http://www.w3.org/International/articles/language-tags/',
diff --git a/core/tests/Drupal/KernelTests/Core/Installer/InstallerLanguageTest.php b/core/tests/Drupal/KernelTests/Core/Installer/InstallerLanguageTest.php
index c7161494fd20..71f542839ef6 100644
--- a/core/tests/Drupal/KernelTests/Core/Installer/InstallerLanguageTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Installer/InstallerLanguageTest.php
@@ -20,8 +20,9 @@ class InstallerLanguageTest extends KernelTestBase {
     // Different translation files would be found depending on which language
     // we are looking for.
     $expected_translation_files = [
-      NULL => ['drupal-8.0.0-beta2.hu.po', 'drupal-8.0.0.de.po'],
+      NULL => ['drupal-8.0.0-beta2.hu.po', 'drupal-8.0.0.de.po', 'drupal-8.0.x.fr-CA.po'],
       'de' => ['drupal-8.0.0.de.po'],
+      'fr-CA' => ['drupal-8.0.x.fr-CA.po'],
       'hu' => ['drupal-8.0.0-beta2.hu.po'],
       'it' => [],
     ];
diff --git a/core/tests/fixtures/files/translations/drupal-8.0.0.fr____CA.po b/core/tests/fixtures/files/translations/drupal-8.0.0.fr____CA.po
new file mode 100644
index 000000000000..e2106954b8b4
--- /dev/null
+++ b/core/tests/fixtures/files/translations/drupal-8.0.0.fr____CA.po
@@ -0,0 +1,4 @@
+# This file exists to prove that
+# \Drupal\Core\StringTranslation\Translator\FileTranslation::findTranslationFiles()
+# does not find it. See
+# \Drupal\KernelTests\Core\Installer\InstallerLanguageTest::testInstallerTranslationFiles()
diff --git a/core/tests/fixtures/files/translations/drupal-8.0.x.fr-CA.po b/core/tests/fixtures/files/translations/drupal-8.0.x.fr-CA.po
new file mode 100644
index 000000000000..e69de29bb2d1