There's been some discussion in the Discord regarding elven dialects. That is, Sithican and Elven: supposedly, a speaker of one can speak or understand the other.
The language tags, however, are separate. So it's my understanding that if you don't have the right tag, you do not speak or understand the language. Similarly, someone's tried to claim they understood Xanalress because they spoke Elven, and the languages have similarities.
Image that was shown by another player:
Furthermore, there's the misunderstanding that, for example, a Darkonese-speaking person can understand Latin, implying they are the same language. A more prominent clarification about this (and the fact that PotM uses real-world placeholder languages) would be welcome.
Only looking for a DM ruling here, since it's been going back and forth.