tinymce-docs-4x/_includes/configuration/language-url.md

##language_url

When using the language option to set the user interface language you may choose to place any language pack(s) in a directory other than the default. If you do this you need to provide a simple URL to where the language file is located. We recommend using a site absolute URL.

Type: String

Example:

tinymce.init({
  selector: "textarea",  // change this value according to your html
  language_url : "/languages/fi.js"  // site absolute URL
});

You can find and download languages here.