OK, it turns out that this is one of a class of known bugs of long
standing (see issue554676 and issue1080864, for example).  The
recommended solution is to not use locale.getdefaultlocale, but to use
locale.getperferredencoding.  I have confirmed that that works for the
case of ca_ES.UTF-8@valencia in python2.5.

There is at least a doc bug here, since no mention of this
fragility/recommendation is made in the getdefaultlocale documentation.

Using getpreferredencoding seems to be the correct solution to your
problem.  However, the locale.py module contains a number of examples of
modifiers in the locale_alias table.  Presumably this case could be
added, but it is not clear to me what the policy is on that at this
time, so I'm adding Martin to the nosy list looking for some guidance.