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

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

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