Translation:Which language's grammar is the most difficult?
"Which language's grammar is most difficult?" should be accepted; the article "the" is superfluous here.
By British English standards, using the is no more proper than omitting it in this sentence according to my understanding. It is just a matter of style.
Same in Australia. I'm a 50 year old native English speaker who's always been interested in languages and grammar and couldn't say which is more common or more standard or more formal.
Agree with Cloud. For eg. Even Spanish omits "I". Learning a language is not just about the language in question. It's also how it is being translated since it is learnt via another language. Be more open minded.
That must be because Arabic is a group of languages spoken across many countries, with the common one being the written variation used in the Koran.
In this context, I feel that they are pretty much the same thing. 哪个 is a shortened form of 哪一个, which literally translates to "which one". 什么 is literally "what".