I am just so confused about this, but I think I have found out why Play Store isn't switching language or updating, I have this is the system log.
07-22 13:53:42.503 1000 1286 1738 I PackageInstallerSession: Session [207568532] was destroyed because of [Installation of com.android.vending version 84191730 is blocked to prevent breaking gmscompat. Max allowed version is 84189999]
So, instead it stays at the outdated English version provided by App Store.
I am not certain how this explains the stalling for app installations and updates I have seen. But one thing seem to have improved after the factory default, it now detects that Play Services is installed and proposes to update it, and it seems that update usually works. So I will set up my phone like it were again. Either factory defaulting has fixed something, or factory defaulting doesn't help.