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the language block (locale and lang_dropdown modules) freaks out a bit.
the behavior is very inconsistent.
eventually the way to fix it, sorta, is to add the language block, set the language, then change the admin_language settings to only work in admin and admin/*, and then configure the language for administration at site and account level
and even though, it has unpredictable results... because the language block will not work on administration pages (admin and admin/*)