Do undead speak any specific language? The ones that can speak that is.

Usually either the ones they could speak in life, or the languages the mage who raises them speaks.

~SE++

What Squinty said. It's worth noting however that most intelligent undead are just that - intelligent. It's not entirely uncommon for particularly old undead to speak more languages then everything the party can speak combined. Draconic, Elven and Abyssal are all fairly commonly spoken among the older undead creatures.

Entirely dependent on the setting!

Pathfinder has an undead specific language called Necril, as a tidbit.

That's actually pretty cool, never knew that.

~SE++

Setting specific, but I usually have ones who can speak knowing the languages of their life + the primary language of the necromancer, whatever that raised them.

Establishing particular, yet I normally have ones that could talk recognizing the languages of their life + the key 5e languages of the necromancer, whatever that elevated them

Depends on if they still have a working tongue. Note that they can write just fine without it, but if the eyeballs decayed the writing is illegible.

i guess i could make a joke about necromancing threads