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This option should contain a language code of the language pack to use with TinyMCE. These codes are in ISO-639-1 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 it back to the project by uploading it as a patch at SourceForge. A description on how to create language packs can be found in the Language Packs section.
tinyMCE.init({
mode : "textareas",
language : "nl"
...
});
Replace "nl" for your language.
Search documentation.