After the upgrade of your Mac to Yosemite Lync 2011 will cease to work.
To fix the issue you need to:
- Open the system preferences and click on Language and Region
- Add English (U.S.) to the language list and drag it on the top of the list
- When you close the system preferences a pop up will inform you that you need to reboot
- After the reboot open the system preferences again and click on Language and Region
- Drag the original language you had on the top before to begin this procedure and reboot again
This should fix the Lync 2011 issue (until Microsoft will release a patch...hopefully)