You gain any one language that any of your favored enemies speak.
When you gain this feature, you also learn one language of your choice that is spoken by your favored enemies, if they speak one at all.
Breaking this down:
- You gain one language.
- It is a language of your choice. You get to pick.
- Its only constraint is that it must be spoken by your favored enemies.
It does not say that you must gain the most common language of your favored enemies. In fact, that would contradict the rule that you get a choice.
Nor can it reasonably be interpreted to be a language that all your favored enemies speak. Many creatures speak only one unique language that is not shared by any others of their type.
Nor does any rule say it has to be a standard language listed in the Player's Handbook.
Therefore, you pick a language that is normally spoken by any creature within your category of favored enemies. If you take Favored Enemy: Monstrosity, you could take Umber Hulk language, although you will not often find it useful, and would be recommended to take the most common language possible.
Obviously, there's some DM judgement here. For example, the bandit entry says it can speak any language, but it's up to your DM what language the bandits of his world speak (probably not an ancient language like Auran, for example) The couatl speaks all languages, but obviously you cannot learn all languages just because you hunt couatl. And just as the favored enemy rules suggest that a ranger should take favored enemies of creatures they have actually encountered, it would be most logical for them to learn the languages of creatures they have encountered. The DM might also impose his own rules about what languages you can and cannot choose.