diff --git a/src/Symfony/Component/Routing/Route.php b/src/Symfony/Component/Routing/Route.php
index 03ec76e0dd..de98e1d763 100644
--- a/src/Symfony/Component/Routing/Route.php
+++ b/src/Symfony/Component/Routing/Route.php
@@ -376,6 +376,10 @@ class Route implements \Serializable
      */
     public function addDefaults(array $defaults)
     {
+        if (isset($defaults['_locale']) && $this->isLocalized()) {
+            unset($defaults['_locale']);
+        }
+
         foreach ($defaults as $name => $default) {
             $this->defaults[$name] = $default;
         }
@@ -418,6 +422,10 @@ class Route implements \Serializable
      */
     public function setDefault($name, $default)
     {
+        if ('_locale' === $name && $this->isLocalized()) {
+            return $this;
+        }
+
         $this->defaults[$name] = $default;
         $this->compiled = null;
 
@@ -461,6 +469,10 @@ class Route implements \Serializable
      */
     public function addRequirements(array $requirements)
     {
+        if (isset($requirements['_locale']) && $this->isLocalized()) {
+            unset($requirements['_locale']);
+        }
+
         foreach ($requirements as $key => $regex) {
             $this->requirements[$key] = $this->sanitizeRequirement($key, $regex);
         }
@@ -503,6 +515,10 @@ class Route implements \Serializable
      */
     public function setRequirement($key, $regex)
     {
+        if ('_locale' === $key && $this->isLocalized()) {
+            return $this;
+        }
+
         $this->requirements[$key] = $this->sanitizeRequirement($key, $regex);
         $this->compiled = null;
 
@@ -577,4 +593,9 @@ class Route implements \Serializable
 
         return $regex;
     }
+
+    private function isLocalized(): bool
+    {
+        return isset($this->defaults['_locale']) && isset($this->defaults['_canonical_route']) && ($this->requirements['_locale'] ?? null) === preg_quote($this->defaults['_locale'], RouteCompiler::REGEX_DELIMITER);
+    }
 }
diff --git a/src/Symfony/Component/Routing/Tests/RouteTest.php b/src/Symfony/Component/Routing/Tests/RouteTest.php
index 6add1337ed..4c767f9d95 100644
--- a/src/Symfony/Component/Routing/Tests/RouteTest.php
+++ b/src/Symfony/Component/Routing/Tests/RouteTest.php
@@ -271,4 +271,65 @@ class RouteTest extends TestCase
         $this->assertEquals($route, $unserialized);
         $this->assertNotSame($route, $unserialized);
     }
+
+    /**
+     * @dataProvider provideNonLocalizedRoutes
+     */
+    public function testLocaleDefaultWithNonLocalizedRoutes(Route $route)
+    {
+        $this->assertNotSame('fr', $route->getDefault('_locale'));
+        $route->setDefault('_locale', 'fr');
+        $this->assertSame('fr', $route->getDefault('_locale'));
+    }
+
+    /**
+     * @dataProvider provideLocalizedRoutes
+     */
+    public function testLocaleDefaultWithLocalizedRoutes(Route $route)
+    {
+        $expected = $route->getDefault('_locale');
+        $this->assertIsString($expected);
+        $this->assertNotSame('fr', $expected);
+        $route->setDefault('_locale', 'fr');
+        $this->assertSame($expected, $route->getDefault('_locale'));
+    }
+
+    /**
+     * @dataProvider provideNonLocalizedRoutes
+     */
+    public function testLocaleRequirementWithNonLocalizedRoutes(Route $route)
+    {
+        $this->assertNotSame('fr', $route->getRequirement('_locale'));
+        $route->setRequirement('_locale', 'fr');
+        $this->assertSame('fr', $route->getRequirement('_locale'));
+    }
+
+    /**
+     * @dataProvider provideLocalizedRoutes
+     */
+    public function testLocaleRequirementWithLocalizedRoutes(Route $route)
+    {
+        $expected = $route->getRequirement('_locale');
+        $this->assertIsString($expected);
+        $this->assertNotSame('fr', $expected);
+        $route->setRequirement('_locale', 'fr');
+        $this->assertSame($expected, $route->getRequirement('_locale'));
+    }
+
+    public function provideNonLocalizedRoutes()
+    {
+        return [
+            [(new Route('/foo'))],
+            [(new Route('/foo'))->setDefault('_locale', 'en')],
+            [(new Route('/foo'))->setDefault('_locale', 'en')->setDefault('_canonical_route', 'foo')],
+            [(new Route('/foo'))->setDefault('_locale', 'en')->setDefault('_canonical_route', 'foo')->setRequirement('_locale', 'foobar')],
+        ];
+    }
+
+    public function provideLocalizedRoutes()
+    {
+        return [
+            [(new Route('/foo'))->setDefault('_locale', 'en')->setDefault('_canonical_route', 'foo')->setRequirement('_locale', 'en')],
+        ];
+    }
 }