This was the case back in Windows XP, so why not in Vista, 7, 8 and 10 that are newer and should have better support? In any case, I would try to add Japanese in your current setup and install the software and see how it works. So if it is a matter of fonts, just install the necessary fonts, although they should install automatically when adding a CJK language (Chinese, Japanese or Korean). And I think the Arial Unicode MS font installed with almost every Office version (can be downloaded separately for free) has the full Unicode range. Well, if I'm not wrong, installing the Japanese Language Pack should also enable the greater Unicode range and complex writing, right-to-left, top-to-down, so there should not be a problem. Also all new Windows versions came first in English and the Greek version was out one or two months later, we just couldn't wait, so we used the English version. I remember some Greek programs did not display correctly in the Greek version (probably due to incorrect font settings) and displayed fine in the English version after we had changed the language for non-Unicode programs to Greek! So some of us more advanced users often avoided the Greek version and used English instead. But in earlier Windows there were some compatibility bugs in the Greek version that were not in the English version. The only advantage using a Greek Windows version is that all settings are automatically set to Greek, so a novice user has nothing to worry about.

Once we change it to Greek and restart all legacy programs display correctly in Greek. My problems usually arise from legacy (non-Unicode) programs that try to display Greek and display garbage because the language for non-Unicode programs (in Regional Settings) is set to English US rather than Greek. I'm not aware though of any program requiring a specific Windows language to install. Here in Greece, we can install the English version and then install the Greek Language Pack if we want Greek interface. If you have done all these and changed the default display language to Japanese, you should be able to install the Japanese version without any issue.

Have you tried to load the Japanese Language Pack (Control Panel -> Languages, Add Language), set your time/date settings to Japanese and your region to Japan? I think this last one, your region, is what makes the version install or not.

Japanese locale emulator win 10