FlashChat is the most internationally-recognized chat system for a reason: the language support is fantastic, and includes multi-byte string support for non-Latin character sets like Chinese, Hindi, Arabic, Cyrillic, and of course the European languages.
If you need to update a language file, be sure to use a good text editor, like Textpad, and save using the UTF-8 file format.
It's fairly straight-forward... just edit as needed, but be mindful that items in ALL_CAPS should remain unchanged, since they are constants. If you are adding a new language, then you should ensure that the 2-letter language code matches what is provided in /inc/config.php.
You can easily disable any language file by commenting out the appropriate line in /inc/config.php. This file is also where you set the default language, which was probably set during installation of FlashChat. In the example below, the Sweedish file is disabled.
For more information about UTF-8 format, and to download a great little editor for the FlashChat language files, please refer to Veronica's post on the forum here: