Yesterday, I switched to polyglot 2 ( a plugin that handles multiple languages in posts ) and it turned out that the change of locale option is broken. Fortunately one of the responses in the authors blog had the answer: add include(ABSPATH . WPINC . '/locale.php'); after load_default_textdomain(); in your polyglot.php and in locale.php of wordpress add global $weekday, $weekday_initial, $weekday_abbrev, $month, $month_abbrev; at the beginning of the file. After that, I had to change my WPLANG to “en” in order to work. Now everything should be working.