I think that the issue that people tend to forget is the effects of deep time. Again, we're talking generation ships - so, we're looking at a thousand years, at least and quite likely more than that.
How much do you have in common with your 1000 year previous ancestor? Culturally? Linguistically? We're talking understanding gaps that would be virtually unbridgeable. In a generation ship, with such a huge paucity of referent points, language would morph almost instantly. Vast swaths of concepts would vanish virtually overnight - weather, animals, anything to do with nature in general. Sky blue? WTF is a sky to someone eight or ten generations removed? Sea green? What's a sea?
Sure, you could have pictures and whatnot, but, within a couple of generations, those images would have zero significance to people. A Butterfly Valve? What's a butterfly? On and on and on.
As it stands, if you could go back in time to talk to someone who shared your language from 1000 years ago, you could barely communicate. Over there, there are ak. Oh, no, I have red eyen. English, going back that far, had separate words for every plural, like we have child-children today. The plural S came from the viking settlers. You really can't directly speak to someone that far back. The language is just too different.