First thing that comes to mind is the video game Tunic, where the objective of the game is to decipher the language.

Secondly, Stargate (the movie) while not entirely or majorly in a fictional language, the alien characters speak their own language consistently, and part of the plot involves how they communicate. In the TV show they forego of that because it would be a pain to have new languages every episode, so you do have to suspend your disbelief for that, but the movie is golden in that regard.

Then there’s other stuff like Sims or Shadow of the Colossus where everything is in made up languages but it has no impact in the plot or mechanics.