Currently there is no way to add any custom languages for the ckeditor languages button. You can only All languages or the UN defined languages. It would be nice to be able to add only some of the languages or if there are languages that you are using that are not in the predefined to be able to use those as well.
If you configure a text format and edit the config for it, there is only currently two options to add languages to the ckeditor languages.
Add a third option call custom languages with the ability to add any or all of the predefined languages as well as the ability to add custom languages that are not part of the predefined languages. A patch will be provided to allow for this option.
Active
1.0
Code