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

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

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