In this example, I will localize v3.0 of an app for Traditional Chinese (zh-hant) using an existing zh-hant strings file from v2.9 of the app.

StringsFileParser.m

#import <Foundation/Foundation.h>
 
int main(int argc, const char * argv[])
{
 
    @autoreleasepool {
        // load the canonical en strings file -- this can be generated with ibtool
        NSDictionary *enStrings = [NSDictionary dictionaryWithContentsOfFile:@"v3.0/en.lproj/MainStoryboard.strings"];
        
        // load a localized strings file from the previous version
        NSDictionary *prevLocalizedStrings = [NSDictionary dictionaryWithContentsOfFile:@"v2.9/zh-hant.lproj/MainStoryboard.strings"];
        
        // using the keys from the en string file, create a new dictionary
        NSMutableDictionary *newLocalizedStrings = [NSMutableDictionary dictionaryWithCapacity:enStrings.count];
        
        for (NSString *key in enStrings) {
            NSString *prevLocalizedStringValue = prevLocalizedStrings[key];
            if (prevLocalizedStringValue) {
                // this is an existing localized string
                [newLocalizedStrings setObject:prevLocalizedStringValue forKey:key];
            }
            
            else {
                // key not found in previous strings file, it's a new localized string for this version
                [newLocalizedStrings setObject:enStrings[key] forKey:key];
                NSLog(@"Key %@ not found in previous localized strings file", key);
            }
        }
        
        [newLocalizedStrings writeToFile:@"v3.0/zh-hant.lproj/MainStoryboard.strings" atomically:YES];
        
        // finally, using the NSLog output in the console, note the new localizations which you need to handle manually
        NSLog(@"Done!");
    }
    return 0;
}