languageNameForLanguageCode:(NSString*)targetLang inLanguageCode:(NSString*)displayLang // this convert language code (ex. en, fr, ja) to language name (ex. English, français, 日本語)

languageName.m

/****
    NSLog(@"%@", [self languageNameForLanguageCode:@"fr" inLanguageCode:@"en"]); // French
    NSLog(@"%@", [self languageNameForLanguageCode:@"ja" inLanguageCode:@"fr"]); // japonais
    NSLog(@"%@", [self languageNameForLanguageCode:@"en" inLanguageCode:@"ja"]); // 英語
****/

- (NSString*)languageNameForLanguageCode:(NSString*)targetLang inLanguageCode:(NSString*)displayLang
{
    CFLocaleRef displayLocale = CFLocaleCreate(kCFAllocatorDefault, (CFStringRef)displayLang);
    CFStringRef displayName = CFLocaleCopyDisplayNameForPropertyValue(displayLocale, kCFLocaleLanguageCode, (CFStringRef)targetLang);
    return CFBridgingRelease(displayName);
}