diff --git a/config/routes/annotations.yaml b/config/routes/annotations.yaml
index 836fcabc..bb85917e 100644
--- a/config/routes/annotations.yaml
+++ b/config/routes/annotations.yaml
@@ -3,3 +3,6 @@ controllers:
     type: annotation
 
     prefix: '{_locale}'
+    requirements:
+        # Match only locales like de_DE or de
+        _locale: "^[a-z]{2}(_[A-Z]{2})?$"