Unit testing Flask Babel translations

I'd like to do some unit tests to check my flask app translations. I have tried this piece of code:

def test_pt_br(self):
    with app.test_request_context():
        app.config['BABEL_DEFAULT_LOCALE'] = 'pt_BR'
        rv = app.test_client().get('/')
        assert 'Execute, melhore' in str(rv.data)

However, it does not work/pass although the app runs fine. What am I doing wrong?