Stick with a modern language. Spanish is a very widely spoken language so will open a lot of doors for you.
Although languages such as Latin are very interesting, they are dead languages and will likely not be useful to you. I would stick to studying dead languages as a hobby rather than doing qualifications in them, as modern languages are more useful and relevant.
**I so want to start learning Middle English but GCSEs in two months, and I’m also learning 6 modern languages so :////