We've got a multi site installation with Gravity Forms. The two sites right now are English and Spanish. When you fill out a form on the Spanish site, and there are validation errors, the form from the English site is loaded in place of the original:
Check the 'email' field when you fill it out. It starts out as "Correo electrónico", but if there are errors on the form, the "Email" label is displayed.
The forms do have the same ID, but they are not part of the same site on the network.
Looking a little closer, it looks like the action on the form is incorrect; it doesn't include the full domain (missing /es):
<form method="post" enctype="multipart/form-data" target="gform_ajax_frame_5" id="gform_5" action="/voluntario/#gf_5">