We've got some interesting language conventions here...
Most of my gamers only know one language. The characters, however, are frequently multi-lingual. Whatever is one to do?
Well, we generally use "Common = modern english without technological references"
For other languages, we then prefix a code-word to denote the language used. We've got Agruk (dwarven), Ishcabibble (elven), Farfegnugen (gnomish), Go'Cha! (orcish), and so on. So, if a character wanted to insult you in dwarven, the player might say, "Agruk: Your mother has the cooking-skill of a doorknob."
Then, on top of that, I'll tend to use archaic English (with thees, thous, and forsooths) to represent a character speaking in somewhat archaic fashion. Frequently, a character speaking inother than his native tongue gets teh archaic treatment. An elf speaking in common, or a dwarf speaking in gnomish, will tend to throw in the older words...