Apparently it was possible to play the game with Japanese audio by changing a line in the .ini settings.
But that was removed in an update.

So, thanks to Olathoë at Steam forums (https://steamcommunity.com/sharedfiles/filedetails/?id=2503724745), I'm uploading an xdelta patch for those who don't know how to operate an hex editor.

https://pixeldrain.com/u/iAJtiQsx

And for those who don't have an xdelta patcher: https://github.com/marco-calautti/DeltaPatcher/releases/latest
You just need to drag and drop the original "raiden5_one.exe" from your GOG folder and the xdelta patch in the respective fields. Then press "Apply patch". No weird dependencies, no CLI.