It worked!?!?!
Well, I just want to report that in my case this solution worked perfectly, even if I didn't have any hope and it doesn't make sense to me.
- Before installing Yosemite, Lync worked.
- Just after installing Yosemite, Lync seemed to be able to connect and then disconnected immediately. (Since I had the option to remember my password checked, it did this cycle until it locked my account... but that's another topic). Closed Lync.
- Using System Preferences, Language & Region, I added a new language to my mac (ENGLISH US) and made it primary. Rebooted.
- Attempted to launch Lync. Still didn't work.
- Using System Preferences, Language & Region, I removed the new language, reverted to my old primary language (ENGLISH). Rebooted.
- Attempted to launch Lync. It works.
- Confused.
Anyway, thanks to everybody who posted this solution. You are the reason there is hope for the web.