Provides a smart LocalizedString() function (short: "LS") that gives you more control over where you pick the translations from

trans.m

// Written 2019 by Thomas Tempelmann, [email protected]
// Tested on macOS, but may work on iOS as well.

NSMutableSet *reportedMissingLocs = nil;

#define MyAppSupportFolderName "your.bundle.id" // or your app's name

static NSBundle *mainBundle = nil;
static NSArray<NSString*> *availableLangs;	// list of langs the app knows (possibly sorted alphabetically)
static NSDictionary<NSString*,NSBundle*> *bundlePerLang = nil;
static NSString *KeyNotFound = @"-Key-Not-Found-";	// has to be unique

static NSString *languageCode (NSLocale *loc) {
	// lang:de-DE becomes lang2:de
	if ([loc respondsToSelector:@selector(languageCode)]) {
		return [loc languageCode];
	} else {
		return [loc objectForKey:NSLocaleLanguageCode];
	}
}

static NSString * appDataPath2(BOOL createIfMissing)
{
	NSString *result = nil;
	NSArray *paths = NSSearchPathForDirectoriesInDomains(NSApplicationSupportDirectory, NSUserDomainMask, YES);
	if (paths.count > 0) {
		result = [paths.firstObject stringByAppendingPathComponent:@MyAppSupportFolderName];
		NSFileManager *mgr = NSFileManager.defaultManager;
		if (NOT [mgr fileExistsAtPath:result] && createIfMissing) {
			[mgr createDirectoryAtPath:result withIntermediateDirectories:YES attributes:nil error:nil];
		}
	}
	return result;
}

// Localization helper, see https://samwize.com/2018/01/23/localization-fall-back-to-base-language/

NSString* LS(NSString *key /*, NSString *comment */)
{
	if (key == nil) {
		return nil;
	}

	if (mainBundle == nil) {
		// setup some data once
		reportedMissingLocs = [NSMutableSet set];
		mainBundle = NSBundle.mainBundle;
		availableLangs = mainBundle.localizations;
		
		// Look for extra localizations in App Support folder
		NSString *appDataPath = appDataPath2(false);
		[[NSFileManager.defaultManager contentsOfDirectoryAtPath:appDataPath error:nil] enumerateObjectsUsingBlock:^(NSString * _Nonnull name, NSUInteger idx, BOOL * _Nonnull stop)
		{
			if ([name.pathExtension isEqualToString:@"lproj"]) {
				NSString *path = [appDataPath stringByAppendingPathComponent:name];
				if ([NSFileManager.defaultManager fileExistsAtPath:[path stringByAppendingPathComponent:@"Localizable.strings"]]) {
					NSString *code = [name stringByDeletingPathExtension];
					if ([availableLangs indexOfObject:code] == NSNotFound) {
						availableLangs = [availableLangs arrayByAddingObject:code];
					}
				}
			}
		}];
		
		// cache each lproj now
		NSMutableDictionary *d = [NSMutableDictionary dictionaryWithCapacity:availableLangs.count];
		bundlePerLang = d;
		for (NSString *lang in availableLangs) {
			NSBundle *b = nil;
			NSString *fname = [lang stringByAppendingPathExtension:@"lproj"];
			NSString *path = [appDataPath stringByAppendingPathComponent:fname];
			if ([NSFileManager.defaultManager fileExistsAtPath:[path stringByAppendingPathComponent:@"Localizable.strings"]]) {
				b = [NSBundle bundleWithPath:path];
			} else {
				path = [mainBundle pathForResource:lang ofType:@"lproj"];
				if (path) {
					b = [NSBundle bundleWithPath:path];
				}
			}
			if (b) {
				d[lang] = b;
			}
		}
	}
	
	NSString *result = nil; // [mainBundle localizedStringForKey:key value:KeyNotFound table:nil];

	// Go over all preferred languages until we find the key
	BOOL hadEN = NO, first = YES;
	NSArray *langs = NSLocale.preferredLanguages;	// All languages the user has chosen in System Preferences
	for (NSString *lang in langs) {
		NSLocale *loc = [NSLocale localeWithLocaleIdentifier:lang];
		NSString *lang2 = languageCode(loc);	// lang:de-DE becomes lang2:de
		NSBundle *b = bundlePerLang[lang2];
		if (b) {
			result = [b localizedStringForKey:key value:KeyNotFound table:nil];
			if (result.length > 0 && result != KeyNotFound) {
				return result;
			}
			if (first) {
				// Log missing language
				if (NOT [reportedMissingLocs containsObject:key]) {
					[reportedMissingLocs addObject:key];
					NSLog(@"Missing translation for lang %@: %@", lang2, key);
				}
			}
			first = NO;
			if ([lang2 isEqualToString:@"en"]) hadEN = YES;
		}
	}

	// Fallback to "en"
	if (NOT hadEN) {
		NSBundle *b = bundlePerLang[@"en"];
		if (b) {
			result = [b localizedStringForKey:key value:KeyNotFound table:nil];
			if (result.length > 0 && result != KeyNotFound) {
				return result;
			}
		}
	}
	
	if (result == KeyNotFound) {
		result = key;
	}

	return result;
}