update docs with supported list, fix language check

docs/userregistry.md

@@ -3,7 +3,8 @@
 #### `HKCU\Software\AmneziaWG\Language`
 
 When this key is set, the UI will try to use the specified language as display language.
+Supported languages: "ca", "cs", "de", "en", "es_ES", "et", "fa", "fi", "fr", "id", "it", "ja", "ko", "nl", "pa_IN", "pl", "pt_BR", "ro", "ru", "si_LK", "sk", "sl", "sv_SE", "tr", "uk", "vi", "zh_CN", "zh_TW".
 
 ```
-> reg add HKCU\Software\AmneziaWG /v LimitedOperatorUI /t REG_SZ /d "en" /f
+> reg add HKCU\Software\AmneziaWG /v Language /t REG_SZ /d "en" /f
 ```

l18n/l18n.go

@@ -42,10 +42,13 @@ func prn() *message.Printer {
 func lang() (tag language.Tag) {
 	keyString, res := services.UserKeyString(LanguageUserKey)
 	if res {
-		keyTag := message.MatchLanguage(keyString)
-		if keyTag.String() == keyString {
-			tag = keyTag
-			return
+		normalizedKeyTag, err := language.Parse(keyString)
+		if err == nil {
+			keyTag := message.MatchLanguage(keyString)
+			if normalizedKeyTag == keyTag {
+				tag = keyTag
+				return
+			}
 		}
 	}
 	tag = language.English