This option should contain a language code of the language pack to use with TinyMCE. These codes are in ISO-639-2 format to see if your language is available check the contents of "tinymce/jscripts/tiny_mce/langs". The default value of this option is "en" for English.
If TinyMCE doesn't have a language pack for your language you could always write your own and contribute this back to this project by uploading it as a Patch at SourceForge. A description on how to create language packs can be found in Language packs section.
Example of usage of the language option:
tinyMCE.init({
...
language : "en"
});