Robustify language code derivation

Author: friedbunny

platform/ios/app/MBXViewController.m

@@ -932,7 +932,7 @@ - (NSString *)bestLanguageForUser
     NSString *bestLanguage;
 
     for (NSString *language in preferredLanguages) {
-        NSString *thisLanguage = [language substringToIndex:2];
+        NSString *thisLanguage = [NSLocale localeWithLocaleIdentifier:language].languageCode;
         if ([supportedLanguages containsObject:thisLanguage])
         {
             bestLanguage = thisLanguage;