babel.core.negotiate_locale

        def negotiate_locale(preferred, available, sep='_', aliases=LOCALE_ALIASES):
    """Find the best match between available and requested locale strings.

    >>> negotiate_locale(['de_DE', 'en_US'], ['de_DE', 'de_AT'])
    'de_DE'
    >>> negotiate_locale(['de_DE', 'en_US'], ['en', 'de'])
    'de'

    Case is ignored by the algorithm, the result uses the case of the preferred
    locale identifier:

    >>> negotiate_locale(['de_DE', 'en_US'], ['de_de', 'de_at'])
    'de_DE'

    >>> negotiate_locale(['de_DE', 'en_US'], ['de_de', 'de_at'])
    'de_DE'

    By default, some web browsers unfortunately do not include the territory
    in the locale identifier for many locales, and some don't even allow the
    user to easily add the territory. So while you may prefer using qualified
    locale identifiers in your web-application, they would not normally match
    the language-only locale sent by such browsers. To workaround that, this
    function uses a default mapping of commonly used langauge-only locale
    identifiers to identifiers including the territory:

    >>> negotiate_locale(['ja', 'en_US'], ['ja_JP', 'en_US'])
    'ja_JP'

    Some browsers even use an incorrect or outdated language code, such as "no"
    for Norwegian, where the correct locale identifier would actually be "nb_NO"
    (Bokmål) or "nn_NO" (Nynorsk). The aliases are intended to take care of
    such cases, too:

    >>> negotiate_locale(['no', 'sv'], ['nb_NO', 'sv_SE'])
    'nb_NO'

    You can override this default mapping by passing a different `aliases`
    dictionary to this function, or you can bypass the behavior althogher by
    setting the `aliases` parameter to `None`.

    :param preferred: the list of locale strings preferred by the user
    :param available: the list of locale strings available
    :param sep: character that separates the different parts of the locale
                strings
    :param aliases: a dictionary of aliases for locale identifiers
    """
    available = [a.lower() for a in available if a]
    for locale in preferred:
        ll = locale.lower()
        if ll in available:
            return locale
        if aliases:
            alias = aliases.get(ll)
            if alias:
                alias = alias.replace('_', sep)
                if alias.lower() in available:
                    return alias
        parts = locale.split(sep)
        if len(parts) > 1 and parts[0].lower() in available:
            return parts[0]
    return None