If using the User Extended field plugin on Joomla 2.5 there is some weird ness with the language files. Attached to this post is a working admin language file. The file has been edited to work with Joomla 2.5.7 and is working on K2 2.5.7
Install it via ftp to administrator/language/en-GB/en-GB.plg_k2_userextendedfields.ini
Note its lowercase whereas original sees to have Camel Case. Also rename it so it ends in 'ini' and not '.txt'