Was it system-wide change (and reboot is done)? It may be needed if python-apt
gets something from outside (e.g. by dbus).
I have no idea, I used update-manager to start the upgrade process and my env:
torkve@yaroot-46:/tmp [2016-10-14 02:01:18]
0 ➜ locale
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Upgrade to yakkety fails due to non-us locale
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