[1.2.X] Fixed #14290 -- Made format localization faster by caching the format...

[1.2.X] Fixed #14290 -- Made format localization faster by caching the format modules. Thanks, Teemu Kurppa and Anssi Kääriäinen for the report and initial patches.

Backport from trunk (r13898).

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13903 bcc190cf-cafb-0310-a4f2-bffc1f526a37