I've been having this same issue ever since installing the latest Microsoft update on Friday, January 26th. I tried several of the methods mentioned in prior posts to switch the lock screen back to English, all to no avail. Yesterday, however, the screen had reverted to English on its own and has stayed that way for the past 2 days.
I'm not sure if Microsoft found a fix, because I don't have the automatic updates feature turned on and I haven't downloaded or installed any new updates since the 26th. It'll be interesting to see if others find the same kind of mysterious solution.