Translation:On źle mówi po angielsku.
Can adverbs like źle, in a sentence like this, only be put in front of the verb?
Or could one also say: "on mówi źle po angielsku", or even: "on mówi po angielsku źle"?
all are possible. but the last one is the least obvious/natural.
Polish has a free word order. Why don't you fix it here?
Polish has a relatively free word order. Please forget the idea of a "free" word order. It's simply not true and such an idea makes some people write totally bizarre sentences.
Having said that, this sentence had correct options missing (ones posted by AndRL82), which I've now added.
does żle means wrong or is it zły (bad) in a different case?
It is an adverb from "zły", but mostly means that someone does something rather poorly. It can be 'wrong': "Źle odpowiedziała" = "She answered wrongly" (?) = Her answer was wrong.
I used zle and received an "incorrect" showing that kiepsko is the proper word. Previously, I have used zle and never received an error. I'm not sure what kiepsko means and what is the difference between these words.
"źle" is the default version, it should have been accepted (well, with a typo).
"kiepsko"... well, the words for "bad" are quite problematic in Polish. Yes, "źle" is the direct translation, but I'd also probably go with "kiepsko". Even though it's colloquial. Still, it very often sounds better than "źle". Similarly a form of "kiepski" often sounds better to me than "zły" (when it means 'bad' and not 'evil' or 'angry').
"kiepski" is like... "baddish". "kinda bad". "not really good". But... it's just often closer to what you want to say. "On mówi źle po angielsku" is more like "His English is just BAD. SO SO BAD." "On mówi kiepsko po angielsku" is more like "His English is... well, not that great". It's hard to pinpoint the exact meaning. So "źle" is stronger, but sometimes it just seems too literal and... too much.