Swift：如何本地化计时器标签

I created a 4:00 minute countdown timer (NStimer). At the end of the countdown the figures change to letters, so instead of showing 0:00, it'll show DONE. I know how to localize buttons, but how do I localize this string, which is only visible in the function below? The string does not appear in the Main.strings. I want to translate it into several languages.I created a 4:00 minute countdown timer (NStime