With respect to language, what about characters from second-world fantasy*? That is, if I were to app Gandalf, he'd be speaking Westron and/or one of the variants of Tolkien Elvish, not any earthly language. While the NPCs would be totally chill with that, I imagine it would be an extreme barrier to roleplaying with others' characters.

* Fantasy not set on Earth. Or any setting where a character can be presumed to not know any Earth languages.