New project fails with USE_I18N=False

[gasman]

Issue Summary

Initial migrations fail on Wagtail 2.11.1 with USE_I18N = False and LANGUAGE_CODE = 'en-us' in settings.

Steps to Reproduce

1. Start a new project with wagtail start myproject
2. Edit myproject/settings/base.py and set USE_I18N = False
3. Run ./manage.py migrate

This throws the exception:

Traceback (most recent call last):
  File "./manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/Users/matthew/.virtualenvs/use18n/lib/python3.7/site-packages/django/core/management/__init__.py", line 401, in execute_from_command_line
    utility.execute()
  File "/Users/matthew/.virtualenvs/use18n/lib/python3.7/site-packages/django/core/management/__init__.py", line 395, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/Users/matthew/.virtualenvs/use18n/lib/python3.7/site-packages/django/core/management/base.py", line 330, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/Users/matthew/.virtualenvs/use18n/lib/python3.7/site-packages/django/core/management/base.py", line 371, in execute
    output = self.handle(*args, **options)
  File "/Users/matthew/.virtualenvs/use18n/lib/python3.7/site-packages/django/core/management/base.py", line 85, in wrapped
    res = handle_func(*args, **kwargs)
  File "/Users/matthew/.virtualenvs/use18n/lib/python3.7/site-packages/django/core/management/commands/migrate.py", line 92, in handle
    executor = MigrationExecutor(connection, self.migration_progress_callback)
  File "/Users/matthew/.virtualenvs/use18n/lib/python3.7/site-packages/django/db/migrations/executor.py", line 18, in __init__
    self.loader = MigrationLoader(self.connection)
  File "/Users/matthew/.virtualenvs/use18n/lib/python3.7/site-packages/django/db/migrations/loader.py", line 53, in __init__
    self.build_graph()
  File "/Users/matthew/.virtualenvs/use18n/lib/python3.7/site-packages/django/db/migrations/loader.py", line 210, in build_graph
    self.load_disk()
  File "/Users/matthew/.virtualenvs/use18n/lib/python3.7/site-packages/django/db/migrations/loader.py", line 112, in load_disk
    migration_module = import_module(migration_path)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
  File "<frozen importlib._bootstrap>", line 983, in _find_and_load
  File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 728, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/Users/matthew/.virtualenvs/use18n/lib/python3.7/site-packages/wagtail/core/migrations/0056_page_locale_fields_populate.py", line 8, in <module>
    class Migration(migrations.Migration):
  File "/Users/matthew/.virtualenvs/use18n/lib/python3.7/site-packages/wagtail/core/migrations/0056_page_locale_fields_populate.py", line 15, in Migration
    BootstrapTranslatableModel('wagtailcore.Page'),
  File "/Users/matthew/.virtualenvs/use18n/lib/python3.7/site-packages/wagtail/core/models.py", line 538, in __init__
    language_code = get_supported_content_language_variant(settings.LANGUAGE_CODE)
  File "/Users/matthew/.virtualenvs/use18n/lib/python3.7/site-packages/wagtail/core/utils.py", line 261, in get_supported_content_language_variant
    supported_lang_codes = get_content_languages()
  File "/Users/matthew/.virtualenvs/use18n/lib/python3.7/site-packages/wagtail/core/utils.py", line 224, in get_content_languages
    (default_language_code, languages[default_language_code]),
KeyError: 'en-us'

This seems to happen because with USE_I18N = False, the implementation of get_supported_language_variant in Django's 'null' i18n backend returns LANGUAGE_CODE unchanged, whereas the real one takes fallbacks into account (returning 'en' for 'en-us'). Wagtail's get_content_languages then expects to find this in settings.LANGUAGES, which fails because Django's master LANGUAGES list contains 'en', not 'en-us'.

• I have confirmed that this issue can be reproduced as described on a fresh Wagtail project: yes

Technical details

• Python version: 3.7.2
• Django version: 3.1.3
• Wagtail version: 2.11.1