There is no official announcement but a Spokesperson from Google Germany has tweeted that official Google+ app for windows phone is on its way. But he did not say anything about the date of release.
Google+ app already has mobile app for Android and iOS devices. Now Google do not want to ignore windows phone users as they are growing and with increasing market share. Google also know that it can not ignore any type of users if it want to compete with Facebook.
Currently users can access Google+ in mobile browser if they do not have Android or iPhone mobile phones.
Windows Phone has very less users so app for this platform is late. But Google cannot ignore Windows Phone platform. So, we can expect the app soon.
If you are Windows Phone smartphone user and like Google+, wait for only few days.
