I'm a DM who has a ranger in my campaign with Undead as her favored enemy. With the new update fixing the bugs we knew we had to reselect her language. However, undead don't have a universal language. Does that mean that it's open to any language? As an experienced DM, I'm going to make my own ruling, but I wanted to get this out in the open in case there is something that I (or other players) am missing or have a similar question.

For my part, I'm going to allow her the choice of whatever language within whatever I feel is reasonable. For instance, should she choose Undercommon, despite never having been to the Underdark, or met anyone from the Underdark, seems implausible and I wouldn't allow it. Celestial, Infernal, or Abyssal seem fine, if she has spent the time researching Orcus, or the like. Other race languages would be backstory dependent, as well.

Just my thoughts. Let me know what you think, or if I missed something.

Presumably your expertise is due to some history with the creature type. So I could see a backstory involving a battle with a necromancer that used undead dwarves or proximity to a haunted elven forest as justification for Dwarven or Elven language proficiency, for example.

That is sort of along the line I was thinking, too. It's easy when the enemies are orcs or goblins or giants. I guess it is another good example of why DMs and PCs should work closely together on some of the finer points of their background. Thanks for the suggestion.