I find feats like this, along with all of the creature type specific ones, are really campaign dependent. You either take them reactively once it becomes clear that it would be really handy to speak, say, Gnomish, or you need to metagame a little bit with your GM to know which languages won't be dud picks.

In my game, I would also find it totally reasonable to retrain a language that hasn't ever shown up, using less downime than it would normally require.