Okay, it seems I made some progress. I was right to think the problem was in the keyboard. I tried these steps:
1) Started the machine with a USB keyboard plugged in: KDM detects the correct locale.
2) Started the machine with both USB keyboard and Logitech K800: KDM is detecting the correct locale for BOTH keyboards.
3) Started the machine with Logitech K800 plugged in: wrong locale again.
So the problem seems to be in keyboards locale detection using the Logitech Unifying Receiver.
I could also reproduce the problem:
1) Installed Oneiric on a new partition with a USB keyboard plugged in. (RIGHT LOCALE)
2) Unplugged the USB keyboard and plugged the Logitech Unifying Receiver. (RIGHT LOCALE)
3) Rebooted to KDM. (RIGHT LOCALE)
4) Upgraded to Precise and rebooted to KDM. (WRONG LOCALE)
Okay, it seems I made some progress. I was right to think the problem was in the keyboard. I tried these steps:
1) Started the machine with a USB keyboard plugged in: KDM detects the correct locale.
2) Started the machine with both USB keyboard and Logitech K800: KDM is detecting the correct locale for BOTH keyboards.
3) Started the machine with Logitech K800 plugged in: wrong locale again.
So the problem seems to be in keyboards locale detection using the Logitech Unifying Receiver.
I could also reproduce the problem:
1) Installed Oneiric on a new partition with a USB keyboard plugged in. (RIGHT LOCALE)
2) Unplugged the USB keyboard and plugged the Logitech Unifying Receiver. (RIGHT LOCALE)
3) Rebooted to KDM. (RIGHT LOCALE)
4) Upgraded to Precise and rebooted to KDM. (WRONG LOCALE)