True, I mean it was how long ago that mankind noticed that stars shift? He wouldn't have noticed and he might not remember even if he did want to (I can't describe the sky to anyone) only when the greeks started to name the constellations that he might have concerned himself with that.

That is a great point which I hadn't realized, with effort an Immortal could learn the new slangs as they come but it would be a great effort and people of the generation wouldn't think to explain it to someone who was there. So leave for a while, see some things and then come back when the friends you had are already history so not to feel fresh pain and on top of that have to deal with an evolving language.

Though in Methos particular case he seems to enjoy language shifts, so he doesn't really count