Use gettext algorithm to determine locale for activity.linfo

When activity bundles are loaded, the activity name and summary are
pulled from an activity.linfo file that is associated with
locale. However, sometimes locale, which uses $LANG has been
superceded by $LANGUAGE as set in
extensions/cpsection/languages/model.py. This patch uses the same
algorithm as gettext.py to find activity.linfo

This problem is seen on the .AU systems which use en_AU for locale and
en_GB for LANGUAGE.