Making languages is way harder than just writing a story, and fantasy isn't trying to be an accurate reflection of real world geo-social stratification. Everyone speaks the same language because it often would be a worse narrative if the characters couldn't communicate. (Not always, but plot-by-misunderstanding is at least as lazy as writing in one tongue)

There needs to be (imo) a reason beyond realism to make that part of the story. Tolkien was exceptional, but he was using different races and languages to make up a creation mythology for the UK. The history and culture and differences were arguably more important than a ring and some hobbits.

Tad Williams also made up a few languages for his Memory, Sorrow, and Thorn series. There they serve to make the world feel bigger, with far-off exotic lands and ancient mysteries.

Oh and you brought up swearing in your first post. De-modernizing language is way harder than it seems. Taking out "god damn" is easy, but that also links to gosh, darn, dang, "goodbye" is from God be with ye, "gossip" from god-sibling, the days of the week all reference Earth myths and have to go, "knight" doesn't make sense unless they had a French equivalent language to take words from...