The language that needs to be in use.
Examples
French
<script src="/Areas/Development/dist/modular/editor/js/messages/messages.fr-fr.js"></script>
<div id="editor">Hover buttons in the toolbar in order to see localized tooltips</div>
<script>
$("#editor").editor({
locale: 'fr-fr'
});
</script>
German
<script src="/Areas/Development/dist/modular/editor/js/messages/messages.de-de.js"></script>
<div id="editor">Hover <b><u>buttons</u></b> in the toolbar in order to see localized tooltips</div>
<script>
$("#editor").editor({
locale: 'de-de'
});
</script>