Will it look bad if I do my home language as an A Level?

If it's your first language, yes, it won't look great. If you take it as an additional A-Level on top of your 4 others, you'll be fine. However, if you aren't a fluent speaker despite it being your home language, take it.

Many people take A levels in their native language I think but from what I've seen universities vary on how they take it. Some don't count them, some do. Don't take it as one of your three to A2 but taking it as an extra one or an AS should be fine, it might limit you a bit (particularly for medicine) but hopefully not too much.

Edit: If you're not fluent then that's probably different, just make it clear on your personal statement when you get that far I guess. Come to think of it, would they even know you were Spanish?

This. I know Bath, and I think Durham too (only examples I've looked at) specifically say they do not accept A Levels in someone's native language, however I'm not sure how strictly the enforce "native" - I mean if you moved to the UK 5 years ago it's pretty obvious but as you're not completely fluent... Maybe they would be fine with it. I'd suggest emailing the tutors of uni's your interested in, or if you don't have a specific uni in mind just take a few high-medium ranked uni's randomly and see their opinions.

Despite this though, I'm relatively sure you'd be fine in taking it to at least AS if not A2 if you wanted - although for medicine probably will want to take first 3 A2 tbh

I really wouldn't say I'm fluent, I came to the UK when I was 5 years old so I wasn't taught how to write or read in Spanish so I'm not sure if I would be an exception? I honestly don't know how I managed to get an A* at the GCSE.