Sega Chief's original NT documentation explains how to rename files to run on European systems. I've included that documentation in my Plus pack, but the process should work for either version.
Edit: OK that documentation isn't quite right. And I'm not 100% on if it works on Switch. But here's the gist of what you should try:
Drill down to to the data folder. Not every mod changes these files so they won't all be there, but if they are you should change them to match your language.
Rename the lang-en folder to lang-fr, lang-es, or lang-de
Open the Field folder and rename flevel.lgp to fflevel.lgp, sflevel.lgp, or gflevel.lgp
in the WM folder rename world_us.lgp to world_fr.lgp, world_gm.lgp, or world_sp.lgp
in the menu folder, menu_us.lgp -> menu_fr.lgp, menu_gm.lgp, menu_sp.lgp
For the handful of cosmetic credits or title screens, look in the CD folder:'
cr_us.lgp -> cr_fr.lgp, cr_gm.lgp, cr_sp.lgp
disc_us.lgp -> disc_fr.lgp, disc_gm.lgp, disc_sp.lgp