i had to change the following code in userform.js in order to be able to add fields to a form in a different locale than the default:

var action = $("#Form_EditForm").attr("action") + '/field/Fields/addfield';

to this:

var orgAction = $("#Form_EditForm").attr("action");
var action = '';
         
if ( orgAction.indexOf( '?' ) != -1 ) {
   action = orgAction.replace( '?', '/field/Fields/addfield?' );
}
else {
   action = orgAction + '/field/Fields/addfield';
}

This makes sure the link for the add field button in the cms is not malformed.