They speak english! Although they have a very limited vocabulary and tend to add a little slang to what they say (troll champion for example Whaaaz up!?" ) , they're not the sharpest tool in the shed! In fact, they're quite stupid!
Whenever they say something that you don't recognize in the english language, they're grunting or making noises!
Here's a link to extend your knowledge on Trolls a little bit! https://tibia.fandom.com/wiki/Trolls
The only actual creature language that you can learn to speak is Orc Language.
Here is a link on wikia regarding orc language: https://tibia.fandom.com/wiki/Orc_Language
Cheers!
Jay~