It's up to you, and only you, of course, but I will say that you'll be absolutely nowhere close to having "learnt a language" if you do one for A level.
I never took a language for GCSE, but having spent a few years (on and off) "studying" Spanish in my spare time I was intrigued, so I looked at a few A level past papers and the activities I could do - the reading and listening - were so unbelievably simple for me it actually genuinely shocked me. I would estimate someone with a decent A2 (European Framework) level would find it quite easy. Anyone who is B1 or higher would find it a breeze.
The past papers I looked at didn't include translation questions, but I would think that'd be a particular strength of mine, and if you are B1+ I can't imagine it posing any problems.
Now, I can't speak (no pun intended) for the speaking exam, but I've heard of people coming out of university degrees in a language who can barely string basic sentences together, so I can't imagine the majority of A level students are speaking with any kind of fluency.
My point is that I doubt the academic approach to learning languages can compare to a self study approach in your spare time that focuses on listening and reading comprehension over grammar study.
I'm 100% positive it's a waste of time if your goal is to understand the spoken language and speak it well yourself. You could probably do that in far less time, and in a far less painful way going it alone.
I was so surprised at the basic level of these papers that I took to these forums to see what students had said about them. Many were saying how tough they'd found it. Some were talking about how they struggled with basic tenses. I mean, I don't know what to say really. How can a student go through 2 years of GCSE and a further 2 years of A level (the top level of school education) and still not know how to conjugate the future tense!!?? Something that anyone can do within a month with a minimal amount of exposure and effort, for ALL the tenses.
I can tell you how: The academic approach is horribly inefficient when it comes to language learning. Students don't get the kind of exposure they need. It's not their fault, but then it kind of is (for some, at least), as I suspect they don't take time away from the course to just listen and read for pleasure. Though they probably don't have much time for it with the workload from other subjects. Or at least they don't prioritise it.
So yeah, unless you're looking to specifically work as a translator or something, where you need qualifications in the language, I really wouldn't bother. Just work on it as a hobby and you'll probably reach a higher listening/speaking level in MUCH less time, if you're passionate about it.
I will say that doing an A level might be a way to spark an interest in languages, but I wouldn't do one of that interest was already there.