For me personally, the most frustrating part of translating Japanese into English is looking up the definitions of words I don't know.
Reading Japanese on its own is a challenging but mostly pleasurable activity. I generally sokudoku (速読, skim/speed-read) and try to understand everything from bunmyaku (文脈, context), looking up words only when I would be missing something fundamental if I didn't know what they meant.
Writing in English is also enjoyable. It's fun to put words on the page, even if it isn't always easy. Looking up a definition when translating forces me away from either of these activities and distracts me from the real matter at hand: production of a new text in a target language.