To be honest, the language requirements for teachers are a bit nuts.
A few years ago, I was interested in teaching functional English in further ed to adults who don't have GCSE/Level 2.
I thought I'd be qualified to do it.
I have a top grade in O level English, a degree in French and German, a PGCE, an ESOL qualification and an MA in linguistics. (Sorry, not boasting) I can be forced to teach any subject in a secondary school, even if I don't want to.
BUT NO!
I'm allowed to teach non-English speakers English, but I can't teach English speakers to read and write their own language up to GCSE level in a college setting. To do that, I need A level English and a further certificate, all of which I would have to pay for myself.