Finally, what I am asking: is this phrase "And this man always starts to speak in language the continuation of which he doesn't know!" understandable, knowing the context? Or, maybe, I should change smth? And what is better, 'the rest of which' or 'the continuation of which'?
Then, is it possible to say just '...is always speaking in languages..." instead of '...always starts to speak languages...'? I just thought, the latter makes the sentence more clear.
But, please, don't propose to change the whole phrase into smth like 'He always speaks in languages he doesn't know properly', because it this case all the irony is lost, I want to preserve 'continuation':)
Thanks to everyone who could at least read this complicated thing till the end:)