FIX Correct Zend_Locale fallbacks in i18n/DateField/DateTimeField

Due to the recent change of translations to transifex, some
locales changed their names, which prompted a fix to
i18n::get_available_translations() (see 00ffe7294).
This caused a regression where short locales are determined
from the YAML file names (e.g. "en"), but weren't matched up
with fully qualified locales from get_available_translations() (e.g. "en_US").
Since this list is used in the admin/myprofile dropdown for the Member.Locale value,
it didn't match up with any entries and defaulted to the first one ("Africaans").

Note that the behaviour of admin/myprofile is still a bit weird:
It defaults the locale on new members to the one set for the current administrator.
So if a site defaults to en_US in _config.php, but the admin happens to view
his backend in de_DE, all members he creates default to de_DE as well.

Thanks to @tractorcow for contributing and peer reviewing!