extendDictionary method
Add translations to the existing Map.
Unlike setMessagesDict, this will not overwrite the existing map, just adds to it (or replace in case the key is duplicated).
Implementation
void extendDictionary({required Map<String, String>? dict, String? lang}) {
lang ??= currentLanguage;
if (dict != null) {
_messages[lang] ??= <String, String>{};
_messages[lang]?.addAll(dict);
}
}