I really need help with creating my ranger. For my "favored enemy," I chose undead, but I don't know what language they speak (if they speak one at all). Do I just look for a single undead (e.g. a Ghoul speaks Common) and say that is what all undead speak? Please help me.
Undead don't have a dedicated language
In general, they speak languages they knew in life. Here's a couple of examples:
Languages understands the languages it knew in life but can't speak
Languages the languages it knew in life
Languages Understands all languages it knew in life but can't speak
There are plane-specific undead like Death Knight which speaks the corresponding language (Abyssal in this case). Most undead usually understand or speak Common.
Since Favored Enemy still has to grant additional language proficiency, the player is usually free to choose any non-secret language people in the game world speak. The exact list depends on the setting and your DM.