@fabio
In file:
plugins/oc-content/plugins/image_required/index.php
Find line:
echo '<input name="ir_image_check" id="ir_image_check" type="hidden" value="0" class="{required:true, min: ' . $min_img . ', messages: { min: \'Upload at least ' . $min_img . ' picture(s)\'}}" />';
Replace it with:
echo '<input name="ir_image_check" id="ir_image_check" type="hidden" value="0" class="{required:true, min: ' . $min_img . ', messages: { min: \'' . __('Upload at least', 'image_required') . ' ' . $min_img . ' ' . __('picture(s)', 'image_required') . '\'}}" />';