The usual answer I give for this sort of question is JLPT N2. You need to be able to recognise around 1000 kanji characters. You also need to understand spoken Japanese in a variety of different social contexts. Otherwise, you'll be able to read the words but the sentences won't make sense.

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I studied Japanese in high school and university. I also spent a lot of time simply reading various Japanese writing materials before I took up translating. It's probably not a good idea to try to learn basic Japanese as you translate, because this makes your progress slower.