What also matters is their mindset, I they have accepted change (as thy probably have over thousands of years) then they will probably also act and talk generally like the people of the current age. The 21 century immortal may not have accepted change yet though, and may have the same mindset as the old man down the street who keeps repeating 'back in my day...' these kinds of immortals would probably speak in a more proper and formal way, preferring not to use slang and popular phrases.