After the upgrade of your Mac to Yosemite Lync 2011 will cease to work.

To fix the issue you need to: 

  1. Open the system preferences and click on Language and Region
  2. Add English (U.S.) to the language list and drag it on the top of the list
  3. When you close the system preferences a pop up will inform you that you need to reboot
  4. After the reboot open the system preferences again and click on Language and Region
  5. 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)