Translation:We will have found him by December.
It means 'want' if the person(s) has a desire to do it, and 'will' if it's something that's certain to happen. I'm not sure if there are any good ways to tell the difference except by immersion.
To me 'vil' would translate to 'will' in more formal contexts, such as some newspapers or articles, while 'kommer til å' can be used in all cases. In most cases 'vil' translates to 'want to'.