A friend of mine suggested an addon idea to me. He wanted an addon that would automatically switch languages upon entering the Demon Hunter Order Hall. So I set about finding out the relevant IDs and here we are, an addon that has two functions:
1. Upon entering The Fel Hammer, swap language to Demonic
2. Upon leaving The Fel Hammer in any way, swap language back to Orcish/Common (depending on your faction, of course)