Sony Xperia Z1 compact w/ Android 5.1.1: how to change locale setting?
Finally I got an answer from Sony's helpdesk: only solution is to reset then re-install the device :( And so I did, and - although this took me quite a bit of time - at least everything behaves as before; too stupid Sony doesn't allow to alter localization settings as others do (like Samsung, HTC, and even plain Android...)