Django/venv/lib/python3.7/site-packages/django/middleware/locale.py

from django.conf import settings
from django.conf.urls.i18n import is_language_prefix_patterns_used
from django.http import HttpResponseRedirect
from django.urls import get_script_prefix, is_valid_path
from django.utils import translation
from django.utils.cache import patch_vary_headers
from django.utils.deprecation import MiddlewareMixin


class LocaleMiddleware(MiddlewareMixin):
    """
    Parse a request and decide what translation object to install in the
    current thread context. This allows pages to be dynamically translated to
    the language the user desires (if the language is available, of course).
    """
    response_redirect_class = HttpResponseRedirect

    def process_request(self, request):
        urlconf = getattr(request, 'urlconf', settings.ROOT_URLCONF)
        i18n_patterns_used, prefixed_default_language = is_language_prefix_patterns_used(urlconf)
        language = translation.get_language_from_request(request, check_path=i18n_patterns_used)
        language_from_path = translation.get_language_from_path(request.path_info)
        if not language_from_path and i18n_patterns_used and not prefixed_default_language:
            language = settings.LANGUAGE_CODE
        translation.activate(language)
        request.LANGUAGE_CODE = translation.get_language()

    def process_response(self, request, response):
        language = translation.get_language()
        language_from_path = translation.get_language_from_path(request.path_info)
        urlconf = getattr(request, 'urlconf', settings.ROOT_URLCONF)
        i18n_patterns_used, prefixed_default_language = is_language_prefix_patterns_used(urlconf)

        if (response.status_code == 404 and not language_from_path and
                i18n_patterns_used and prefixed_default_language):
            # Maybe the language code is missing in the URL? Try adding the
            # language prefix and redirecting to that URL.
            language_path = '/%s%s' % (language, request.path_info)
            path_valid = is_valid_path(language_path, urlconf)
            path_needs_slash = (
                not path_valid and (
                    settings.APPEND_SLASH and not language_path.endswith('/') and
                    is_valid_path('%s/' % language_path, urlconf)
                )
            )

            if path_valid or path_needs_slash:
                script_prefix = get_script_prefix()
                # Insert language after the script prefix and before the
                # rest of the URL
                language_url = request.get_full_path(force_append_slash=path_needs_slash).replace(
                    script_prefix,
                    '%s%s/' % (script_prefix, language),
                    1
                )
                return self.response_redirect_class(language_url)

        if not (i18n_patterns_used and language_from_path):
            patch_vary_headers(response, ('Accept-Language',))
        response.setdefault('Content-Language', language)
        return response