Against either candlepin hosted or standalone, set your LANG env var to de_DE and run subscription-manager-cli register with a bad username/pass combo (for standalone you'll have to edit your config to include an auth line).

Results:
UnicodeEncodeError: 'ascii' codec can't encode character u'\xfc' in position 23: ordinal not in range(128)

Expected:
A nice message in german about invalid username/password.

Note that the gui handles this fine, as does the log entry.