Interface Localization
The languages files must be placed inside the "editor/lang" directory of the project. You can use the file "en.js" as the base file for your translations. The file names are based on the 3066 standards (but in lowercase) and the 639-1. So, for example, a file for the Portuguese language should be called "pt.js".
Translation could also be localized for a specific country. In this case, for example, a Brazilian Portuguese translation file must be called "pt-br.js."
The browser’s language auto detection system uses the first available language that best fits the client settings. For example, if a client computer is set to Brazilian Portuguese and there is no pt-br.js file available, the pt.js is used if available. If still not available, the default language is used (from the configuration file, by default English).
When creating new language files, you need to add an entry for it in the "editor/_source/internals/fcklanguagemanager.js" file. Something like this:
AvailableLanguages = { en : 'English', pt : 'Portuguese' }
To test your language file, you could run the editor using the source scripts. Just open the "_samples/html/sample03.html" files.
When you feel the translation is completed and the tests are done, you will need to "re-generate" the compressed JavaScript files of FCKeditor using the FCKpackager