I came across your article about WonderSwan development in 2020 and have been toying around with thing.
I'm leaving this to let you know that in Windows 10 you can change the language for non-unicode programs.
Open Settings > Time & Language > Language > Administrative language settings > Language for non-Unicode programs > Set to Japanese.
While it's not much of difference if you can't read the language, it might still help with searching documentation for translations.