diff --git a/src/Symfony/Component/Translation/Tests/TranslatorTest.php b/src/Symfony/Component/Translation/Tests/TranslatorTest.php
index 742956c0a9..b7d2d1cd18 100644
--- a/src/Symfony/Component/Translation/Tests/TranslatorTest.php
+++ b/src/Symfony/Component/Translation/Tests/TranslatorTest.php
@@ -50,7 +50,7 @@ class TranslatorTest extends TestCase
     {
         $translator = new Translator($locale);
 
-        $this->assertSame($locale, $translator->getLocale());
+        $this->assertSame($locale ?: (class_exists(\Locale::class) ? \Locale::getDefault() : 'en'), $translator->getLocale());
     }
 
     /**
@@ -91,7 +91,7 @@ class TranslatorTest extends TestCase
         $translator = new Translator($locale);
         $translator->setLocale($locale);
 
-        $this->assertEquals($locale, $translator->getLocale());
+        $this->assertEquals($locale ?: (class_exists(\Locale::class) ? \Locale::getDefault() : 'en'), $translator->getLocale());
     }
 
     /**
diff --git a/src/Symfony/Component/Translation/Translator.php b/src/Symfony/Component/Translation/Translator.php
index 5e5d9570ca..c36ae331ba 100644
--- a/src/Symfony/Component/Translation/Translator.php
+++ b/src/Symfony/Component/Translation/Translator.php
@@ -143,6 +143,7 @@ class Translator implements LegacyTranslatorInterface, TranslatorInterface, Tran
         }
 
         $this->assertValidLocale($locale);
+        $locale ?: $locale = class_exists(\Locale::class) ? \Locale::getDefault() : 'en';
 
         $this->resources[$locale][] = [$format, $resource, $domain];
 
@@ -163,7 +164,7 @@ class Translator implements LegacyTranslatorInterface, TranslatorInterface, Tran
         }
 
         $this->assertValidLocale($locale);
-        $this->locale = $locale ?? (class_exists(\Locale::class) ? \Locale::getDefault() : 'en');
+        $this->locale = $locale;
     }
 
     /**
@@ -171,7 +172,7 @@ class Translator implements LegacyTranslatorInterface, TranslatorInterface, Tran
      */
     public function getLocale()
     {
-        return $this->locale;
+        return $this->locale ?: (class_exists(\Locale::class) ? \Locale::getDefault() : 'en');
     }
 
     /**
@@ -281,7 +282,7 @@ class Translator implements LegacyTranslatorInterface, TranslatorInterface, Tran
      */
     public function getCatalogue($locale = null)
     {
-        if (null === $locale) {
+        if (!$locale) {
             $locale = $this->getLocale();
         } else {
             $this->assertValidLocale($locale);
@@ -505,7 +506,7 @@ EOF
      */
     protected function assertValidLocale($locale)
     {
-        if (null !== $locale && 1 !== preg_match('/^[a-z0-9@_\\.\\-]*$/i', $locale)) {
+        if (!preg_match('/^[a-z0-9@_\\.\\-]*$/i', (string) $locale)) {
             throw new InvalidArgumentException(sprintf('Invalid "%s" locale.', $locale));
         }
     }