Determines the user's locale from ``Accept-Language`` header.

        See http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.4

def get_browser_locale(self, default: str = "en_US") -> tornado.locale.Locale:
        """Determines the user's locale from ``Accept-Language`` header.

        See http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.4
        """
        if "Accept-Language" in self.request.headers:
            languages = self.request.headers["Accept-Language"].split(",")
            locales = []
            for language in languages:
                parts = language.strip().split(";")
                if len(parts) > 1 and parts[1].startswith("q="):
                    try:
                        score = float(parts[1][2:])
                    except (ValueError, TypeError):
                        score = 0.0
                else:
                    score = 1.0
                locales.append((parts[0], score))
            if locales:
                locales.sort(key=lambda pair: pair[1], reverse=True)
                codes = [l[0] for l in locales]
                return locale.get(*codes)
        return locale.get(default)