the "plonejsi18n" view canot create correct language code when request 's LANGUAGE contain sub-language

[adam139]

the "plonejsi18n" view is responsive for getting tranalated vocabulary through ajax request.
When the browser request's LANGUAGE contain sub-language,for example:"zh-CN",i18n.i18njs will generate wrong language code: "zh-cn",This correct language code should be "zh_CN".
`
def call(self, domain=None, language=None):

    if domain is None:
        catalog = {}
    else:
        if language is None:
            language = self.request['LANGUAGE']
        catalog = self._gettext_catalog(domain, language)

    response = self.request.response
    response.setHeader('Content-Type', 'application/json; charset=utf-8')
    response.setBody(json.dumps(catalog))
    return response`