The Forgotten Server is written in C++, which controls most of the in-game variables and algorithms.
Lua is the scripting language for Tibia OTs and provides you the ability to create in-game things like new quests, events, spells, etc.
The only other language that is worth mentioning is XML, which I believe they are attempting to move away from. Mostly is just used for the monsters.xml index file and various other indexes like vocations, promotions, etc.
Other than that, there are a few web services that are provided as well. Those are most likely going to be written in PHP.
EDIT: Didn't realize this post is old as fuck. Came up on a suggested post when I was reading another.