[luxpir]

As a general rule it probably depends on what you're trying to translate. If it matches the corpus well, you'll do better. This is simplified, of course, as NMT is a bit of a black-box. But we know the GIGO concept still applies.

Perhaps Japanese has a smaller corpus to draw from. Perhaps less work has been done in that pair. Perhaps you're just not translating the right kind of text. It's hard to say.

Comparing Japanese to Chinese is not really apples to apples. Likewise for European languages. Sharing a writing system in any form (Latin/Roman or Chinese) doesn't really come into it.