Well based on your A-level choices it seems like you're quite interested in languages. I was also offered the choice to study an additional language at uni, in either first or second year, although not part of my degree like you, it was just for a certificate. i chose not to as i thought it would take up too much time but looking back, i regret not doing so now as my degree is quite light and an extra language always helps. I'd therefore say do some research into your chosen degree, how much time it requires, modules etc, if you know the actual uni you want to go to then perhaps try and find someone on here who is studying your chosen degree there and see how they're coping time wise. If you think you can manage it though i'd say go for it as an additional language always gives you an advantage.