Lync actually uses the current input language for the spell checking.
You can thus choose the language as follows:
- Add all input languages that you want to use in Windows' Input Language settings (combined with your usual keyboard layout)
- Open Lync
- Change input language in the Task Bar
The only problem is that the input language is per-application so it applies to all conversions, even if you disable the tabbed mode. I guess there is nothing we can do against that.
Found via https://wlmail.wordpress.com/spelling0/