I have been trying to get lync to work on Yosemite since first beta release. I finally have it working thanks to the comments from posters in this forum. I tried the change of language on DP7 and lync immediately started working.
1 go to setting "regional language prefs"
2. Move second place language in the list to the top (by dragging it up list)
3. Restart
4. Return original language to top of list
5. Restart
6. lync now working
I have since upgraded this morning to DP8 and everything still working.
I am connecting to a company lync 2010 server, using OS X client 14.0.9