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.

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git-svn-id: http://core.svn.wordpress.org/trunk@27463 1a063a9b-81f0-0310-95a4-ce76da25c4cd