"Which language's grammar is the most difficult?"

Translation: 什么语言的语法最难？

[NaturalStream]

I don't know. The Chinese doesn't look right to me. How about 哪个语言的语法是最难的？ or 最难的语言语法是哪(一)个？

[KX3.]

Your first translation is indeed more idiomatic than the other two (given and your second one).

[Ronios]

The classifier for languages is 种. It's currently accepted too.

[Elder-Simmons]

And the answer is??