(You forgot an い at the end of むずかしい, but this is not the main point of my answer).
Let's try to analyze your attempt. You say 勉強は hence you are making 勉強 the topic, which kind of makes sense given what you want to express.
However, what follows does not really make much sense. In fact, if you try to make a grammatical analysis of the rest of the sentence, what is 日本語 here? There is no particle nor any other structure to precisely identify its role in the sentence. In other words, your try is grammatically wrong.
What you probably want to do is to connect 日本語 with the 勉強 in order to express "The study of the Japanese language", that can be done using の:
Now the topic is actually "The study of the Japanese language".
The rest of the sentence is quite OK. Just add むずかしい to describe it as difficult and then add the "but" part. I probably would use けど here rather than でも. It sounds more natural and conversational to me, but I am not a native speaker so let's wait for other comments.
Anyway, putting all together your sentence becomes:
(You probably can add the よ in the end for a little emphasis).
Another possibility, though maybe it goes a bit far from your attempt, is to change it this way：
Or as above with けど、 instead of 。でも、
I put the other first as I tried to stay close to your original attempt.