If you fail the English Language exam in Y10, then you will resit it in Y11 (either in October or June). Some schools will allow you to complete both English Language and Literature together, however most will see Literature as an extension course from Language, and so may be reluctant to allow you to take it without a passing grade (by this I mean a C or above). English Language is of course more important than Literature as it is a core GCSE such as Maths or Science, which allows you to branch out, and also gives you the right first step in towards your future.
I took Lang. at GCSE and got a C, and yet in Lit I ended up with an A.
Realistically, Language is more important, it is the one that opens up gateways to other opportunities in life. However, if you are considering a university course to do with Literature, then obviously do Literature. It is mainly about your future plans. The safe option is Lang. But don't be afraid to take Lit.