It wouldn't help. The real problem is that they aren't comfortable with English. The reason English has all the documentation, all of the methodology, all of the literature, all of the community: network effects.

A Japanese Stack Overflow would inevitably be less useful than access to the English Stack Overflow.

This, exactly. Stackoverflow does have Japanese version, and there are sites like Qiita for programmers to share knowledge. But the problem is most of the existing discussions are in English, and a Japanese version of the sites wouldn't help much except all contents are translated (not possible).