Flask test setup with Flask-Babel

I'd like to setUp with unittest module.

My Flask App is created using factory (create_app) uses Flask-Babel for i18n/

def create_app(config=None, app_name=None, blueprints=None):
    # Create Flask App instance
    app_name = app_name or __name__
    app = Flask(app_name)
    app.config.from_pyfile(config)

    configure_hook(app)
    configure_blueprints(app, blueprints)
    configure_extensions(app)
    configure_jinja_filters(app)
    configure_logging(app)
    configure_error_handlers(app)
    configure_cli(app)

    return app

create_app function calls configure_extensions(app) which is as follows:

def configure_extensions(app):
    """Initialize Flask Extensions."""
    db.init_app(app)
    babel.init_app(app)
    csrf.init_app(app)

    @babel.localeselector
    def get_locale():
        # If logged in, load user locale settings.
        user = getattr(g, 'user', None)

        if user is not None:
            return user.locale

        # Otherwise, choose the language from user browser.
        return request.accept_languages.best_match(
            app.config['BABEL_LANGUAGES'].keys())

    @babel.timezoneselector
    def get_timezone():
        user = getattr(g, 'user', None)

        if user is not None:
            return user.timezone

It works fine when I run app, but I can't create a unittest properly because it asserts error like this:

File "C:\projects\rabiang\venv\lib\site-packages\flask_babel\__init__.py", line 127, in localeselector
'a localeselector function is already registered'
    AssertionError: a localeselector function is already registered

Due to the message "a localeselector function is already registered", I thought that fact that my setUp method of unittest was invoked when each test method is called makes problem. Thus, I changed @classmethod setUpClass like this:

# -*- coding: utf-8 -*-
import unittest

from app import create_app, db
from app.blueprints.auth import auth
from app.blueprints.forum import forum
from app.blueprints.main import main
from app.blueprints.page import page


class BasicsTestCase(unittest.TestCase):
    @classmethod
    def setUpClass(cls):
        blueprints = [main, page, auth, forum]

        app = create_app(config='../test.cfg', blueprints=blueprints)
        cls.app = app.test_client()

        db.create_all()

    @classmethod
    def tearDownClass(cls):
        db.session.remove()
        db.drop_all()

    def test_app_exists(self):
        self.assertFalse(BasicsTestCase.app is None)


if __name__ == '__main__':
    unittest.main()

However, @babel.localeselector and @babel.timezoneselector decorator doesn't work.