How to say in Czech: How good is his Serbian?

English
How good is
Czech
Jak dobře umí srbsky?
His Serbian is getting better.
His Serbian is very good.
How do you say that in Serbian?
I do not speak Serbian very well.
You speak Serbian very well.
His native language is Serbian.
Is he learning Serbian on his own?
He is learning Serbian.
He is not Serbian.
He is Serbian.

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