Django internalization - changing current language does not work

Question

I want to localize my REST app, and have read the doc. Seems like I've done everything accordingly, and yet can't get the message in my exception appear in Russian on client side.

The view:

@api_view(['POST'])
def foo(request):
    if serialized.is_valid():
        activation_key = hashlib.sha1(str(random.random())).hexdigest()[:5]
        email = request.data['email']
        user_language = 'ru'
        translation.activate(user_language)
        request.session[translation.LANGUAGE_SESSION_KEY] = user_language
        try:
            # SMTPRecipientsRefused error generating code
        except SMTPRecipientsRefused:
            return Response({'msg': _('Invalid email address'),
                             'object': None}, status=status.HTTP_400_BAD_REQUEST)
     # ...

And here's what I have done to tackle localization.

1. Set up gettext for Windows downloading the corresponding files from this source
2. Ran the django-admin compilemessages command and successfully generated the .po files
3. Opened the generated .po file and filled in the necessary translation
4. Edited the middleware to include all necessary elements

The .po file looks like this:

# SOME DESCRIPTIVE TITLE.
# Copyright (C) YEAR THE PACKAGE'S COPYRIGHT HOLDER
# This file is distributed under the same license as the PACKAGE package.
# FIRST AUTHOR <EMAIL@ADDRESS>, YEAR.
#
#, fuzzy
msgid ""
msgstr ""
"Project-Id-Version: PACKAGE VERSION\n"
"Report-Msgid-Bugs-To: \n"
"POT-Creation-Date: 2017-01-04 19:49+0400\n"
"PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
"Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
"Language-Team: LANGUAGE <[email protected]>\n"
"MIME-Version: 1.0\n"
"Content-Type: text/plain; charset=UTF-8\n"
"Content-Transfer-Encoding: 8bit\n"
"Plural-Forms: nplurals=4; plural=(n%10==1 && n%100!=11 ? 0 : n%10>=2 && n"
"%10<=4 && (n%100<12 || n%100>14) ? 1 : n%10==0 || (n%10>=5 && n%10<=9) || (n"
"%100>=11 && n%100<=14)? 2 : 3);\n"

#: .\rest\views.py:46
msgid "Invalid email address"
msgstr "Некорректный почтовый адрес"

The middleware:

MIDDLEWARE_CLASSES = [
     #...
    'django.contrib.sessions.middleware.SessionMiddleware',
    'django.middleware.locale.LocaleMiddleware',
    'django.middleware.common.CommonMiddleware',
]

Any help on what I am missing here ?