I haven’t used any plug-ins to translate or localize my theme, I have used .mo and .po files generated by poedit, a stand alone application. When changing the WPLANG codes it affects the site language, not just the WP admin interface. That is as I’m trying to explain the issue here.
By the way, here is a sample from the WP localization guide:
“you’ll need to make sure your wp-config.php file matches your language files. For example, if you’re using a French translation, you’ll need to add the fr_FR.po and fr_FR.mo files to your theme folder, then set your language in wp-config.php, like this:
define (‘WPLANG’, ‘fr_FR’);
So, according to this if you change the WPLANG language code the translation will be affected. That doesnt correspond with my first install cause I had a Norwegian WLPANG with an English WP admin language. My localization files were nb_NO.mo and nb_NO.po and the language in the wp-config.php was (‘WPLANG’, ‘nb_NO’); I had an English program version of WP and the interface was English. That all changed after I updated as mentioned. I had a look at the wp-config.php file and it’s unchanged.
So, it all worked fine before the update and it didn’t after the update. Beats me. If I change the (‘WPLANG’, ‘nb_NO’); to (‘WPLANG’, ‘en_US’); it affects the site language, in other words the localization. Just as the above localization guide suggests.