How to say in Czech: How good is their Japanese?

English
How good is
Czech
Jak dobře umějí japonsky?
Their Japanese is getting better.
Their Japanese is very good.
How do you say that in Japanese?
I do not speak Japanese very well.
You speak Japanese very well.
Their native language is Japanese.
Are they learning Japanese on their own?
He is Japanese.
He is learning Japanese.
He is not Japanese.

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