I also hate this. Epic fantasy in general is very unwilling to grapple with a realistic level of linguistic diversity, partially because it’s easier for writers and partially because most of those writers are themselves monolingual and are used to Anglophone dominance.

Another peeve of mine is epic fantasy worlds where there are in fact multiple human languages, but all non-human species only have a single language. Elvish, Dwarfish, and so on. Like many other epic fantasy problems, this is also explainable as writers trying and failing to achieve Tolkien. (Who, for example, showed the dialectical differences between different forms of Westron and constructed multiple distinct Elven languages.)