I used JavaScript because I'm most comfortable with it - I believe you can use any language you want, but you're better off if it's one the interviewer knows (Java/C for back end, JavaScript for front end.)

I personally applied as a front end engineer, but the only difference is that the system design interview question gets swapped with a front end focused one. Within Google my official title is still "Software Engineer" and I'm expected to work across the stack.