Upgrade to raring fails with UnicodeDecodeError ( 'ascii' codec can't decode byte 0xd0) when using russian locale

When user with russian locale launches ubuntu-release-upgrader, it calculates changes and prompts user to continue installation. This looks like:

Установка обновления может занять несколько часов (trans: installation could take a few hours). После завершения
загрузки отменить установку будет нельзя (trans: after finishing download installation can't be aborted).

 Продолжить [дН] Подробности [п] (trans: Continue [...], Details [...])

Notice that confirmation keys are translated too. It could be a problem of translation, but in previous versions of ubuntu (9.10 -> 10.04) keys д/Н/п worked without any problem.

Traceback (most recent call last):

File "/tmp/ubuntu-release-upgrader-jt3c4s/raring", line 10, in
<module>
sys.exit(main())

File
"/tmp/ubuntu-release-upgrader-jt3c4s/DistUpgrade/DistUpgradeMain.py",
line 240, in main
if app.run():

File
"/tmp/ubuntu-release-upgrader-jt3c4s/DistUpgrade/DistUpgradeController.py",
line 1765, in run
return self.fullUpgrade()

File
"/tmp/ubuntu-release-upgrader-jt3c4s/DistUpgrade/DistUpgradeController.py",
line 1722, in fullUpgrade
if not self.askDistUpgrade():

File
"/tmp/ubuntu-release-upgrader-jt3c4s/DistUpgrade/DistUpgradeController.py",
line 970, in askDistUpgrade
self.cache.required_download)

File
"/tmp/ubuntu-release-upgrader-jt3c4s/DistUpgrade/DistUpgradeViewText.py",
line 166, in confirmChanges
if res.strip().lower().startswith(_("y")):

UnicodeDecodeError: 'ascii' codec can't decode byte 0xd0 in position
0: ordinal not in range(128)