Determines which language definition to use and returns it.  With no parameters, it will return the global language.  If you pass in a language key, such as 'en', it will return the definition for 'en', so long as 'en' has already been loaded using moment.lang.
function getLangDefinition(key) {
        if (!key) {
            return moment.fn._lang;
        }
        if (!languages[key] && hasModule) {
            try {
                require('./lang/' + key);
            } catch (e) {
                // call with no params to set to default
                return moment.fn._lang;
            }
        }
        return languages[key];
    }