I feel obligated to point out that there are 7,200 languages in use today, which means that on average each language is spoken by only 1.1 million people.

In reality, though, that average is incredibly skewed by a handful of megalanguages like English and Mandarin. The majority of languages have just several thousand speakers, and these languages are concentrated in areas where large nation-states have not developed—the types of societies which are common in fantasy novels.

So, unless you’re prepared to conjure up a language or six for every distinct community in your books, you’re not going to achieve a realistic level of linguistic diversity. Good conlanging is hard, and most fantasy authors have little exposure to linguistics. Heck, Tolkien himself was highly educated in linguistics, and even Middle Earth is not realistically linguistically diverse.

If I can suspend disbelief enough to tolerate dragons, I can suspend disbelief enough to tolerate the equally scientifically implausible idea of a “common tongue”. Linguistic sophistication is not a requirement for the genre.

That said, linguistic diversity and realism can add depth to a world in a way little else can, so I do love to see it, just like I enjoy any great deep world-building!