If I add multiple languages pack to my forum, but let say I just add portuguese.
Then, let say we put pt lang as the default forum language.
After that on my user account (admin or not doesnt matter), we select english language on UCP.
Some variables (I bet its all the common.php) still appear as portuguese (the default language) when it shouldn't!
EDIT: I've found that if I comment the line:
from root/phpbb/user.php, the bug dissapear.
Example on the forum: