The title of the main window in Microsoft Skype for Business should always display "Skype for Business" in the language that you selected during Microsoft Office setup.
With that in mind, assume that you set the default language to German. However, when you start the client, the title bar displays "Skype for Business" in Russian instead of German.
To fix this issue, install the March 8, 2016 update (KB3114831) for Lync 2013 (Skype for Business).
Note Lync 2013 was upgraded to Skype for Business in April 2015.