I've figured out how to use some of the English files on the Japanese version of the game (game ID: 01003c101454a000). Obviously, all DLC stuff is still untranslated (and don't count me on translating anything, I only know how to mix and match files), but it still removes some of the burden in remembering cards (and 1 thing that does get translated on DLC cards is type, which definitely helps. No need to remember any of the non-effect DLC monsters). I've also put the English menu files in, so some menu stuff is also translated (only the ones that are text, not the images). The font is pretty ugly though, nothing I can do about that.
This is a LayeredFS mod. Here's how to use it on Ryujinx (for anything else, you'll have to figure it out yourself):
1. Right-click the installed Japanese Rush Duel game
2. Select "Open Mods Directory"
3. Unzip the downloaded zip file into the folder that opens. You can rename the EnglishText folder if you want (but everything else has to keep their name)
EDIT: 1 minor problem I've discovered is that the fusion tutorial
breaks: in the multi-choice effect window you can't select 1 of Sevens Paladin's effects. However, multi-choice fusions still work properly in duels, so there's nothing to worry about.