"I go to China in order to improve my Chinese."
In this case, the reason your suggested sentence is not correct is because you had 「...我去了中國」with the completed aspect marker 了 whereas the English sentence indicates an action that is not yet complete "I go to China ...".
Otherwise, if you had the same sentence but instead had 「...我去中國」then it would be fine, and that answer is accepted.
It's a part of the construct 是为了.
Expressing "the Reason Why"
Sometimes you will see 为了 following 是. In this case, 为了 mean "for." Note that 为了 and 因为 both have the character 为 in them, and they have sort of similar meanings. You will also see 因为 following 是, instead of 为了. It literally means "is because (of)."
Literally, 提高 means "raise" instead of "improve". so you can't.
ps: In Chinese it's always better to put the adverbial before what it modifies. The following sentence is more natural:
If you really want to keep the English word order, you can use the conjunction word 以 from Classical Chinese. But that is too much for beginners:
And in practice this sentence still sounds a bit weird and nobody will know what you mean. So you'd better use "I went to China" or "I'll go to China".