I logged a ticket with WPML that has been escalated to their compatibility team. They advise me to mention this to you, and ask for further support.
At WPML i called the ticket “Invalid login language switching”, and it has been treated by one Shekhar Bhandari.
The issue : on http://188.8.131.52/heinpoblome/
go to another language than english;
go to the ‘my login’ page (‘mon profil’ in french or ‘mijn login’ in dutch);
login with a non-existant user;
the page will switch to the ENGLISH invalid login page;
This is when WPML diagnosed this as a compatibility issue. I will also log this ticket with Formidable, since there does not seem to be a clear ‘culprit’.
there seems to be another dimension to this problem, regarding language switching and widgets.
when i put the Formidable shortcode in a text box on a page, the language switching works properly; I just mean the language switching of the page that shows the login shortcode; i am not referring to what happens when logging in with a unknown user mentioned above.
when i put the shortcode into a widget on a page, the language switching does not work anymore.