Impossible to create new groups of users in a non-English Wiki

To reproduce:

• go to admin section of the Wiki and set under "General" the "Default language" from "en" to e.g. "de"
• now go to the groups section
• try to create an new group of users

Result: A new group cannot be created. You can press the "Create" button in the dialog but nothing happens. When you now go back and set the Wiki Default Language back to "en" you can create new groups without problems.