tornado.locale.Locale.get_closest() not getting closest match

The tornado.locale.Locale.get_closest() (and as such by extension tornado.locale.get) function is a bit malformed. It tries to match two character language codes directly against the frozenset in which the five character language codes are stored, which obviously fails. As a result it can only get exact matches and will return the default locale when using two character codes.

    @classmethod
    def get_closest(cls, *locale_codes):
        """Returns the closest match for the given locale code."""
        for code in locale_codes:
            if not code:
                continue
            code = code.replace("-", "_")
            parts = code.split("_")
            if len(parts) > 2:
                continue
            elif len(parts) == 2:
                code = parts[0].lower() + "_" + parts[1].upper()
            if code in _supported_locales:
                return cls.get(code)
            if parts[0].lower() in _supported_locales:
                return cls.get(parts[0].lower())
        return cls.get(_default_locale)

Specifically this part:

            if parts[0].lower() in _supported_locales:
                return cls.get(parts[0].lower())

I wrote the following simple function in my own code to bypass this problem but I bet someone else can write it a bit nicer into the intended function:

locale = self.request.headers.get('Accept-Language')

if locale:
    for l in tornado.locale.get_supported_locales():
        if locale == l.split("_")[0]:
            self.locale = tornado.locale.get(l)

Start at tornado.locale.Locale.get_closest(), especially the fallback check against _supported_locales. Verify that a two-character language code selects the corresponding supported locale instead of the default locale, and add regression coverage for that behavior.