More translation cleanups.

Affects widgets (see #27112), custom headers (see #21785), theme installer (see #27055, reverts [27614]), and some media stuff. Untranslates some complicated strings that need additional study.

see #27453.


git-svn-id: https://develop.svn.wordpress.org/trunk@27620 602fd350-edb4-49c9-b593-d223f7449a82