Languages in RPGs have a lot of potential, but they often get relegated to the realm of soulless mechanics, or even hand waved completely. Looking at novels, we see that languages are used as inspiration, flavor, and to instill a sense of wonder.
Languages are mysterious:
If no one in the group knows a language, that language and what it holds is a mystery. This is especially true if it’s resistant to magic or skill based attempts to make it understandable. You probably shouldn’t place necessary clues in a magic and skill resistant language, but the very presence of such a thing makes for excellent window dressing and can form the base of a stimulating puzzle.
Languages are magical:
Languages in RPGs don’t have to be just different ways of talking. What about a language that’s actually a viral alien species in your sci-fi game or a language only able to be read by the criminally insane for your horror game?
Languages make your world more vivid:
Languages, even languages that don’t form a barrier to your group’s communication are excellent ways to add flavor and backdrop to your world. Picking a small list of adjectives that describe a language and using them to describe the “musical elven tongue” or the “brutal orcish pidgin” when they come up is an excellent way to make them easier to imagine, without much effort.
Languages identify culture:
In the real world, languages are more exclusive than inclusive. Even if you speak a second language, your accent belies your status as an outsider. This makes guarding language and using your native tongue a matter of pride, not an inconvenience to be tossed aside the instant you write “common” on your character sheet.