.desktop file

In order to find your language support plugin, KDE needs a .desktop file with some meta information.

X-KDE-Library=... this must be the same as the name of your library, i.e. look into the CMakeLists.txt for your kde4_add_plugin line and use the same name
X-KDE-PluginInfo-Name=... this must be the same as the name of your plugin in the code, i.e. look into your main plugin .cpp file and find the K_EXPORT_PLUGIN line, more exactly the first argument to the KAboutData.

After installing your plugin, make sure to run kbuildsycoca4. For testing, try duchainify or ktraderclient.