Steps to Reproduce
- Login as admin;
- Go to Control Panel > Configuration > Instance Settings > Miscellaneous;
- Under Default Language, select any language other than English (United States) and save the change;
- Now, under Available Languages, move all the languages (except the one you previously chose) from the Current pile to the Available pile and save the change;
- Go to the Site's Content > Forms;
- Click to add a new Form;
- Add any field and a title to the Form;
- Click on Save.
The message "your request failed to complete" appears and the Form is not created.
The Form should be successfully created.
Tip: if you're not familiar with any language on the list of default languages besides English (United States), you should first move one that you know, it can be English (Australia) ou English (United Kingdom), from the Available pile to the Current pile and save the change. With that you should able to continue the steps of reproduction.