You can set it in your preferences on web, idk if that syncs to the mobile app. Mine also turned into spanish on web the other day for some reason, which is my secondary language in chrome but my primary language is set to English. In Lemmy settings i just forced it to English
idk if your instance is any different but on lemmy.world I just got to settings on the top right corner where my user name is and set "Interface Language" to English