I'm not sure. I have thought I should not expect it because of the technical
requirements of Kanji, but then I seem to recall recently seeing some mention
of Atari ST users groups here.
Well, the question should be what TOS Japanese users normally use. I would expect US TOS to be the best fit, since I think most modern Japanese keyboards follow the US layout closely.
The default symbolic mappings for -_ [{ }] and `~ were broken for US TOS for a very long time, I made a PR about fixing them last August. All but -_ were accepted, because an argument was made that German TOS needs it assigned differently. Because that was refused is why I began to undertake the per-TOS mappings.