I think the basic dedication is great, and useful. Unfortunately, it locks you in to picking two more Linguist feats if you ever wanted to take a different archetype, and I’m not really a fan of the rest of it.

It’s a fun and useful archetype, like GolGolarion mentioned, but I personally wouldn’t take it unless I’m playing a campaign where I know it’ll come up a lot, and/or we’re playing with Free Archetype. It’s just not worth the class feat slots otherwise.

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.