spellchecker_languages

This option lets you specify a list of languages for the user to select from. The format is based on the 3.x spellchecker format.

tinymce.init({
...
spellchecker_language: 'English=en,Danish=da,Dutch=nl,Finnish=fi,French=fr_FR,' +
'German=de,Italian=it,Polish=pl,Portuguese=pt_BR,Spanish=es,Swedish=sv'
});