Maybe if you replace all Japanese "message", "message_hud", "hud_LZ.bin" and "message_JP_LZ.bin" files with files inside EU_English, or US_English, here is a link whit all system titles:is it possible to change the menu text from say a JP n3ds to EN?
https://www.3dbrew.org/wiki/Title_list#00040010_-_System_Applications
Or some how change the region.