USE_I18N=False causes TypeError when attempting to access Wagtail admin

[MisterTJB]

Expected behaviour

Wagtail StreamForms should be resilient to projects that set USE_I18N=False in the main project settings or the installation docs should include a caveat that users of Wagtail StreamForms should set USE_I18N=True

Actual behaviour

Accessing the admin UI raises a TypeError:

TypeError at /admin/wagtailstreamforms/form/
__str__ returned non-string (type __proxy__)

Switching USE_I18N to True fixes the error.

Steps to reproduce the behaviour

1. Create a new project with Wagtail and Wagtail StreamForms (I'm using Django-3.1.5, wagtail-2.11.3 and wagtailstreamforms-3.13)
2. In settings/base.py set USE_I18N=False
3. Access the admin UI in a browser (doesn't matter if you're logged in or not)
4. TypeError 💥