Luma Locale Switcher won't help.
So, go to GodMode9, enter sysNAND SD > title > 00040000 > TID Low (for that video app) > *.app > Mount as NCCH image/drive
(Check its Title ID with FBI)
Go to romfs folder and Copy "lang" folder to SD card,
Put in sdmc:\luma\titles\<TitleID>\romfs,
Inside lang folder, rename JP_Japanese to EU_English or US_English
(Do that on PC as it's easier)
Reboot 3DS and hold Select to open up Luma config, Enable game patching, save then you're done.