Set default system locale

[gavinelder]

Background

Set a system locale to prevent errors such as this due to system locale falling back to use the language setting on devices which don't inherit default locale from the system.

  File "/usr/local/lib/python3.7/locale.py", line 587, in getlocale
    return _parse_localename(localename)
  File "/usr/local/lib/python3.7/locale.py", line 495, in _parse_localename
    raise ValueError('unknown locale: %s' % localename)
ValueError: unknown locale: en-US

Further to this

Removed logic using LANGUAGE_CODE = locale.getdefaultlocale()[0] as locales as languages are no longer supported with django.
https://github.com/pope1ni/django/blob/5dabb6002ed773c150da4bd3aee756df75d218c2/tests/check_framework/test_translation.py#L23-L31