How to use the l18n.set_language function in l18n

def decorated_view(request, *args, **kwargs):
        user = request.user

        if user.is_anonymous:
            return reject_request(request)

        if user.has_perms(['wagtailadmin.access_admin']):
            preferred_language = None
            if hasattr(user, 'wagtail_userprofile'):
                preferred_language = user.wagtail_userprofile.get_preferred_language()
                l18n.set_language(preferred_language)
                time_zone = user.wagtail_userprofile.get_current_time_zone()
                activate_tz(time_zone)
            if preferred_language:
                with override(preferred_language):
                    return view_func(request, *args, **kwargs)
            else:
                return view_func(request, *args, **kwargs)

        if not request.is_ajax():
            messages.error(request, _('You do not have permission to access the admin'))

        return reject_request(request)