silverstripe-translatable/code/controller/TranslatableEditorToolbarExtension.php

<?php
use SilverStripe\ORM\DataExtension;

class TranslatableEditorToolbarExtension extends DataExtension
{
    public function updateLinkForm(&$form)
    {
        $field = new LanguageDropdownField('Language', _t('CMSMain.LANGUAGEDROPDOWNLABEL', 'Language'));
        $field->setValue(Translatable::get_current_locale());
        $field->setForm($form);
        $form->Fields()->insertBefore($field, 'internal');
        Requirements::javascript('translatable/javascript/HtmlEditorField.Translatable.js');
    }
}