No you only need to know English. What happens is you get a document with the translation and the access to the raw pages and you just gotta make it make sense. it's actually hard since you gotta make the sentences flow, have correct punctuation and sometimes replace whole words and change sentences so much that they seem different yet have the same meaning for that purpose of making it flow. Also you got to be good at picking out appropriate sfx/onomatopoeia
I believe translator needs to have at least a proper English level. Direct translation only makes other people’s work harder, and we have Google translate for that. However very often scanlation groups run into the same old problem of being short on translator, and they may take in someone without sufficient level of English (or the language that they are translating into).
We all know that every scanlation teams has one or two proof readers. I just wanna ask, if they're the proof reader for jpn-English manga translation, does it mean that they can actually understand japanese? (let's say that the proof reader is a non-japanese). Or is it sufficient if they know English only?