They shouldn't forget the language if they forget the skill, that'd be lame!

"Sorry, I don't do bookbinding anymore, and because of that, I have no clue how to speak your language anymore."

You can get a language with 15 lessons or so into Bookbinding. You could forget and learn a few times, and have them all, then pick something else.

That'd be lame too. Cheaters!

It could be a one-off thing, the language skill. So once you learn the language it gets dropped entirely, and blocked if you ever relearn. That way you could maintain the language, since you've learnt it, and you wouldn't be able to cheat to get them all.

On the other hand, you could argue that you forget bookbinding, and everything linked with it. Since language is tied so closely, you 'voluntarily' forget the languages, to 'free space' for other things.

It's the same, really, as me having done Alchemy my entire life, and now I don't have the slightest clue how to do it.

Yeah, I suppose you could look at it like that.

One could say that you can understand it but lack the physical verbal skills to speak it. For example, when I was in Italy and Greece with an Italian tour guide and a Greek bus driver, each spoke their own language: the Italian would say something in Italian, and the Greek would respond using Greek. They were both adept at understanding the other's language, but they were both unskilled in actually verbalizing the languages they understood, moreso from not having learned through repetition the muscular motion patterns you need to do in your mouth to produce the sequences of sounds that are language.

Mmhm, I can understand Japanese when my grandfather speaks to me but I'm not able to say much.

Richter: *says something that makes a decent point*
Roark: *says the exact same thing, uses bigger words*
Everyone else: Go Roark!

He always does that!

But the context was more specific than just that.
Knowing the language (being able to understand it and speak it), and then, because of some kind of accident, forgetting how to speak it but still being able to understand is what seems unlikely to me.
But I'm not familiar with psychology, so maybe it's possible.

My "uncle" (more like... great uncle? great great uncle? Something like that...) suffered a stroke. He wasn't able to speak after that, but he could still understand... from what I was told, this was a huge loss to him because prior to that he was always speaking, and now all he could do is listen.

He wasn't able to speak at all, not just in one particular language, I understand?
Such a selective breakdown is what I find unlikely, not a total loss of ability to speak which I know that happens.