I actually experienced it myself 2 or 3 times, but I cannot reproduce it. To me it happened after a hard reset, at the first two or three reboots, but then disappeared. Apparently at the beginning/after fresh install, the system pulled the local settings/language from the SIM network (I have no other explanation for it), but as I said, it disappeared for me.
Actually: I figured now how to reproduce it.
To me it happens when I wait for appr. half a minute or longer after reboot. When the phone asks me to enter my password, and I ignore it for the above mentioned time, then it suddenly switches to the language of the country I live in (i.e. where the SIM is registered or logged in).
My assumption is that if you don't log in, the phone doesn't load your user settings, but overwrites it with the locales it pulls from the network.
If I reboot again (without logging in), it starts again first in my set language, and after the waiting period it switches again to the local language.
So how can I avoid it? By logging in quickly after the reboot
Edit 2: When not logging in on time, it not only resets the language, but also sound. While I usually have turned off keyboard sound, after that waiting period, when it switches language, it also turns on sound. This underlines my assumption that if not logged in, the phone doesn't load the user settings, but switches to default & pulls the locales from the SIM network.
@Jeremie & @jaezcurra
Could you maybe try that as well, to see if an immediate login after reboot solves the issue? And can you have a look if also the phone suddenly switches language when logging on to the phone?