Actually there's no clue of how we can speak this language, but according with TibiaWiki there's some interesting facts that lead us know that the bonelord language could be somehow related to math:
They are found in crypts: referring to cryptography
Bonelords use eyes to communicate: referring to binary code
The name "bonelord" is a reference to Barebane
In Hellgate there's a formation of skulls that can be thought as a mathematical matrix, so mathematical matrix could be a key to understand the language