If I was in your position, what I'd do is contact some schools in Spain, and ask if you could go over there for a week or two and gain some experience as a teaching assistant!
Completely unimportant. Only matters if you're applying to somewhere like oxbridge I guess as you need pretty much everything to set yourself apart from other candidate
I don't think it's a disadvantage if you don't have it, as long as you can make up for it with other language related things, put it that way. I had none when I applied and was fine with offers. It'd show that you're keen definitely and that you're open to trying new things but I don't think it's essential.
Hi! What kind of other language-related things did you mention? Especially because of COVID it’s so hard to find anything I could do as work experience