It depends a little on how you are launching the editor.

If you're using javascript to launch the editor, then there is a language parameter, see this page: Popup Installation or Embedded Installation If you're just trying to launch the editor within a browser, then you can add the 'lang=fr-rf', as in

http://www.codecogs.com/components/eqneditor/editor.php?lang=fr-fr

obviously use the language code you you require.