WP8.1 Calendar - Birthdays from Facebook show up twice (in 2 languages)

All Birthdays in my Calendar show up twice - once in English (under the color set under Settings for Facebook) and once in German (System Language in a colour set for "Geburtstags-Kalender (Birthday Calender)).

Funny thing is since i didn't enter all these Birthdays manually and the only other source for birthdays was and still is Facebook, it's obvious that what Windows Phone now lists under Birthday Calender is actually data that has been imported from Facebook at some time. Before the Update to 8.1 all Birthdays only showed up once and only in German.

One more thing: I briefly switched to English (US) to try out Cortana and switched back again, don't know if the problem existed before that.

I'm on a L920 with both the Facebook Beta and the normal App installed and linked to my live-Account.

Now, apart from the fact that this is annoying, i'd like to get a clue where it comes from, if anyone else has the same problems and how i can change it back - any suggestions?