diff --git a/src/Symfony/Component/Translation/Formatter/MessageFormatter.php b/src/Symfony/Component/Translation/Formatter/MessageFormatter.php
index c5fd2cea6466..75dfc01c3caf 100644
--- a/src/Symfony/Component/Translation/Formatter/MessageFormatter.php
+++ b/src/Symfony/Component/Translation/Formatter/MessageFormatter.php
@@ -34,7 +34,7 @@ public function __construct(TranslatorInterface $translator = null, IntlFormatte
     /**
      * {@inheritdoc}
      */
-    public function format(string $message, string $locale, array $parameters = [])
+    public function format(string $message, ?string $locale, array $parameters = [])
     {
         if ($this->translator instanceof TranslatorInterface) {
             return $this->translator->trans($message, $parameters, null, $locale);
diff --git a/src/Symfony/Component/Translation/Formatter/MessageFormatterInterface.php b/src/Symfony/Component/Translation/Formatter/MessageFormatterInterface.php
index b85dbfd115c1..6ad8e809d9a5 100644
--- a/src/Symfony/Component/Translation/Formatter/MessageFormatterInterface.php
+++ b/src/Symfony/Component/Translation/Formatter/MessageFormatterInterface.php
@@ -26,5 +26,5 @@ interface MessageFormatterInterface
      *
      * @return string
      */
-    public function format(string $message, string $locale, array $parameters = []);
+    public function format(string $message, ?string $locale, array $parameters = []);
 }
diff --git a/src/Symfony/Component/Translation/Loader/ArrayLoader.php b/src/Symfony/Component/Translation/Loader/ArrayLoader.php
index 0758da8b59a5..03aa14515031 100644
--- a/src/Symfony/Component/Translation/Loader/ArrayLoader.php
+++ b/src/Symfony/Component/Translation/Loader/ArrayLoader.php
@@ -23,7 +23,7 @@ class ArrayLoader implements LoaderInterface
     /**
      * {@inheritdoc}
      */
-    public function load($resource, string $locale, string $domain = 'messages')
+    public function load($resource, ?string $locale, string $domain = 'messages')
     {
         $resource = $this->flatten($resource);
         $catalogue = new MessageCatalogue($locale);
diff --git a/src/Symfony/Component/Translation/Loader/FileLoader.php b/src/Symfony/Component/Translation/Loader/FileLoader.php
index 60c5b348e8af..86aa8b7f7e41 100644
--- a/src/Symfony/Component/Translation/Loader/FileLoader.php
+++ b/src/Symfony/Component/Translation/Loader/FileLoader.php
@@ -23,7 +23,7 @@ abstract class FileLoader extends ArrayLoader
     /**
      * {@inheritdoc}
      */
-    public function load($resource, string $locale, string $domain = 'messages')
+    public function load($resource, ?string $locale, string $domain = 'messages')
     {
         if (!stream_is_local($resource)) {
             throw new InvalidResourceException(sprintf('This is not a local file "%s".', $resource));
diff --git a/src/Symfony/Component/Translation/Loader/IcuDatFileLoader.php b/src/Symfony/Component/Translation/Loader/IcuDatFileLoader.php
index 6052ded55d87..4a59492b5b84 100644
--- a/src/Symfony/Component/Translation/Loader/IcuDatFileLoader.php
+++ b/src/Symfony/Component/Translation/Loader/IcuDatFileLoader.php
@@ -26,7 +26,7 @@ class IcuDatFileLoader extends IcuResFileLoader
     /**
      * {@inheritdoc}
      */
-    public function load($resource, string $locale, string $domain = 'messages')
+    public function load($resource, ?string $locale, string $domain = 'messages')
     {
         if (!stream_is_local($resource.'.dat')) {
             throw new InvalidResourceException(sprintf('This is not a local file "%s".', $resource));
diff --git a/src/Symfony/Component/Translation/Loader/IcuResFileLoader.php b/src/Symfony/Component/Translation/Loader/IcuResFileLoader.php
index dda9284ba4ad..14e7f7f5117a 100644
--- a/src/Symfony/Component/Translation/Loader/IcuResFileLoader.php
+++ b/src/Symfony/Component/Translation/Loader/IcuResFileLoader.php
@@ -26,7 +26,7 @@ class IcuResFileLoader implements LoaderInterface
     /**
      * {@inheritdoc}
      */
-    public function load($resource, string $locale, string $domain = 'messages')
+    public function load($resource, ?string $locale, string $domain = 'messages')
     {
         if (!stream_is_local($resource)) {
             throw new InvalidResourceException(sprintf('This is not a local file "%s".', $resource));
diff --git a/src/Symfony/Component/Translation/Loader/LoaderInterface.php b/src/Symfony/Component/Translation/Loader/LoaderInterface.php
index 2073f2b24ace..cbc4a91c7d95 100644
--- a/src/Symfony/Component/Translation/Loader/LoaderInterface.php
+++ b/src/Symfony/Component/Translation/Loader/LoaderInterface.php
@@ -34,5 +34,5 @@ interface LoaderInterface
      * @throws NotFoundResourceException when the resource cannot be found
      * @throws InvalidResourceException  when the resource cannot be loaded
      */
-    public function load($resource, string $locale, string $domain = 'messages');
+    public function load($resource, ?string $locale, string $domain = 'messages');
 }
diff --git a/src/Symfony/Component/Translation/Loader/QtFileLoader.php b/src/Symfony/Component/Translation/Loader/QtFileLoader.php
index f5cdec697261..ebd62d816141 100644
--- a/src/Symfony/Component/Translation/Loader/QtFileLoader.php
+++ b/src/Symfony/Component/Translation/Loader/QtFileLoader.php
@@ -27,7 +27,7 @@ class QtFileLoader implements LoaderInterface
     /**
      * {@inheritdoc}
      */
-    public function load($resource, string $locale, string $domain = 'messages')
+    public function load($resource, ?string $locale, string $domain = 'messages')
     {
         if (!stream_is_local($resource)) {
             throw new InvalidResourceException(sprintf('This is not a local file "%s".', $resource));
diff --git a/src/Symfony/Component/Translation/Loader/XliffFileLoader.php b/src/Symfony/Component/Translation/Loader/XliffFileLoader.php
index df90978742ad..b81fc693a7bd 100644
--- a/src/Symfony/Component/Translation/Loader/XliffFileLoader.php
+++ b/src/Symfony/Component/Translation/Loader/XliffFileLoader.php
@@ -28,7 +28,7 @@ class XliffFileLoader implements LoaderInterface
     /**
      * {@inheritdoc}
      */
-    public function load($resource, string $locale, string $domain = 'messages')
+    public function load($resource, ?string $locale, string $domain = 'messages')
     {
         if (!stream_is_local($resource)) {
             throw new InvalidResourceException(sprintf('This is not a local file "%s".', $resource));
diff --git a/src/Symfony/Component/Translation/Translator.php b/src/Symfony/Component/Translation/Translator.php
index 006c8991ac42..9da6677d70f4 100644
--- a/src/Symfony/Component/Translation/Translator.php
+++ b/src/Symfony/Component/Translation/Translator.php
@@ -119,12 +119,10 @@ public function addLoader(string $format, LoaderInterface $loader)
      *
      * @param string $format   The name of the loader (@see addLoader())
      * @param mixed  $resource The resource name
-     * @param string $locale   The locale
-     * @param string $domain   The domain
      *
      * @throws InvalidArgumentException If the locale contains invalid characters
      */
-    public function addResource(string $format, $resource, string $locale, string $domain = null)
+    public function addResource(string $format, $resource, ?string $locale, string $domain = null)
     {
         if (null === $domain) {
             $domain = 'messages';
@@ -248,7 +246,7 @@ protected function getLoaders()
     /**
      * @param string $locale
      */
-    protected function loadCatalogue(string $locale)
+    protected function loadCatalogue(?string $locale)
     {
         if (null === $this->cacheDir) {
             $this->initializeCatalogue($locale);
@@ -260,7 +258,7 @@ protected function loadCatalogue(string $locale)
     /**
      * @param string $locale
      */
-    protected function initializeCatalogue(string $locale)
+    protected function initializeCatalogue(?string $locale)
     {
         $this->assertValidLocale($locale);