The naming of the language files for your plug-in is critical if you want the files to be registered correctly with the Joomla! back end.
The plug-in I have recently been building is a modification of the content search plug-in that allows you restrict which article categories are searched. The language file for the administration area is named as follows;
en-GB.plg_search_contentRestricted.ini
A run through of the file name structure can be found in the Joomla! forums, the basics are as follows,
[ISO language code].plg_[extension type]_[extension name].ini
Additional underscores cause problems with Joomla! installing the file, you won’t tend to see any explicit errors, but the problem will be evident as your admin form labels and descriptions will fail to load correctly.