How to say in Czech: Their Japanese is getting better.

English
is getting better.
Czech
Jejich japonština se zlepšuje.
I do not speak Japanese very well.
You speak Japanese very well.
How good is their Japanese?
Their Japanese is very good.
Their native language is Japanese.
Are they learning Japanese on their own?
He is Japanese.
He is learning Japanese.
He is not Japanese.
Is he Japanese?

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