Is sentence correct?

This is a very awkward sentence. I don't understand what you are trying to say, but my guess is that "he" started learning Japanese as a hobby.

I will take a shot.

He changed his hobby because he realized that Japanese is very important for his work.

((I think this works if the sentences before this one explain that Japanese is a new hobby)).

Correct, but not graceful--and a bit confusing out of context.

Hobbies are usually for personal entertainment rather than sharpening professional skills.

He decided to neglect his hobbies and concentrate on learning Japanese which would be very important for his career.

Thanks, guys.

I agree that the sentence is not correct, but I was told that it was fine.