I do n't think anybody here is speaking of literally forgetting language. For me if I am very symptomatic (foggy,fatigued, zombiesh, derealized, some dp, etc) it becomes hard to communicate. I can be a very fluid person and usually am in control of my vocabulary, but when feeling poorly I grasp for words, forget what I am talking about, have trouble following what is being said, and thus may fear responding, If I am very symptomatic I get more depersonalized and will actually lose a sense of what I truly believe or who I am, so again it is difficult to express. When I am "normal" all this does not happen. When engaged in conversation while feeling lousy it actualy seems to bottom out, goes nowhere, becasue I lose a sense of attachment to the subject. If your enviroment feels wierd and your innards are temporarily not connected I can see how one would feel uncomfortable in speech. Indeed folks I am with do not really notice that this is happening, or if they do think I am just tired. It is at this point I go home and go to sleep or divert to something else. Of course the language is still there, and you can still ride the bike. It just seems that sometimes one temproarily does not ride with clarity. And all of this can happen without focusing on the problem. Of course one feels aware, but the hypervigalence you speak of does not have to be present for this to happen. It is part of the gamut of other symptoms. For me it has such a physically drainingl component that it is hard to even think let alone focus. I liken it to studying for a final. Up all night and stressed and haven't eaten and you get to the test and can't even comprehend the questions. Yes, you know the words, but they do not connect.
It is real. It is like trying to talk to someone while stoned.