MKVToolNix.
An excellent tool for working with video files as long as you're okay with your files being in MKV containers (you should be they're superior to mp4). From within it you can add and remove tracks such as audio and subtitle, change flags (flag subs as forced, default, etc), rename tracks for clarity, adjust track timing with positive and negative delays to fix sync issues. You can do batch scripting against it to iterate through whole folders of video files. For anyone who has a home media server it's a must. For anyone who works with videos regularly it's at least useful.