diff --git a/src/main/java/com/yolo/keyborad/utils/RequestLocaleUtils.java b/src/main/java/com/yolo/keyborad/utils/RequestLocaleUtils.java
index 1ae16f8..72b7642 100644
--- a/src/main/java/com/yolo/keyborad/utils/RequestLocaleUtils.java
+++ b/src/main/java/com/yolo/keyborad/utils/RequestLocaleUtils.java
@@ -19,15 +19,13 @@ public final class RequestLocaleUtils {
             return defaultLanguage();
         }
 
-        int separatorIndex = preferredLanguage.indexOf('-');
-        if (separatorIndex < 0) {
-            separatorIndex = preferredLanguage.indexOf('_');
+        String normalizedTag = preferredLanguage.replace('_', '-');
+        Locale locale = Locale.forLanguageTag(normalizedTag);
+        String resolvedTag = locale.toLanguageTag();
+        if (!StringUtils.hasText(resolvedTag) || "und".equalsIgnoreCase(resolvedTag)) {
+            return normalizedTag;
         }
-
-        String language = separatorIndex > 0
-                ? preferredLanguage.substring(0, separatorIndex)
-                : preferredLanguage;
-        return language.toLowerCase(Locale.ROOT);
+        return resolvedTag;
     }
 
     private static String defaultLanguage() {