I noticed that the new Plex App defaults to use the keyboard language, rather than the display language in Windows. I really want to be able to set the app to use English.
The workaround was to find all different locale folders and remove everything by en, en_GB, en_US stuff. Still, looks horrible as some items, like news and playlists are still localised.
Have you checked out the settings? Settings > Plex > General > Language
That’s where I can change the app language in the new Plex Player for macOS. As that’s in line with e.g. Plex Web I suppose it should be the same on Windows (however I cannot confirm this at the moment).