Your command of the language has improved a lot.

[Silverobama]

Are these two sentences natural?

a) Your command of the language has improved a lot.
b) You’ve become much more proficient in the language.

 

[emsr2d2]

They're OK. In real life, "the language" would probably be replaced by the name of the actual language.
Note that I've changed your thread title. You used "mastery of the language", a phrase that appears nowhere in your post!

 

[Skrej]

Probably even more natural is to drop 'command of the language' and just name the language.

Your Tlingit has improved a lot.