Hi! I personally did french GCSE, which I got a 9 in. However, despite me not speaking french, my relatives are french, so whenever I needed help at home I had someone who knew the language. I suppose this makes my opinion on the difficulty therefore unimportant since I effectively had a tutor.
I would recommend a language. A language at GCSE is often wanted, my school required me to do one so I had no choice. I wont lie and say it was a good class, learning languages at school is not the most fun of things, but it's manageable.
A tip I would give, learn tenses first and foremost - they will literally void your answer if you do not use the right tense, and learn set phrases with good grammar in them (you can ask your teacher to write them for you) and then just parrot them in the speaking and put them down on the writing. Then if you panic in the speaking, you say a beautifully constructed phrase about your favourite author, not "umm." like I always did (oops.)
Also, know the quirks of each language. German is hard to pronounce, french is confusing, Spanish is hard to decipher (imho). I would hesitate to do an art, if I can give unwanted advice. Everyone who did an art who did not want to do the A-level regretted it. It takes a lot of your time. If you're doing an art and a language GCSE time will be more heavy.
Then again, you know yourself well. What language are you considering? What are your other options? Are you leaning towards double or triple science? It's good to consider the overall picture of the GCSEs. Good Luck!