In i18nManager.js, XDate locales is initialized by translated month and day name.
But $.t("…") function cannot translate at this mooment because loading i18next's traslation file doesn't finish.
According to i18next documentation, we must add callback function as 2nd argument for processing after loading done.
i18next documentation: http://i18next.com/pages/doc_init.html
So it seems that the solution is using callback function as below.