(that is Lync 15.0.4535.1002 and MSO 15.0.4517.1504) the natural question arises:
How do I get Lync spellcheck in my mother tongue?
Well, to start with, make sure you have the minimum updates for the Lync client. If not, you can download them here: Lync 2013 update 15.0.4535.1510.
This is an updated update of the client "Cumulative Update 2" which will take the client to version Lync 15.0.4535.1507 / MSO 15.0.4517.1504. (Which is kind of weird considering the name of the update, but anyway...)
First install the updates for MSO (KB2727096), MSORES (KB2817624) and IDCRL (KB2817626) which basically are Office 2013 updates and finally install the actual Lync client update lyncloc2013 (KB2825630)
Lync 2013 is a part of Office 2013 and will use the spellcheck feature from Office so you need to find and download an Office 2013 Language Pack for your language. In my case I downloaded "Office 2013 Single Language Pack (x86 and x64) - DVD (Swedish)" and clicked through the setup.exe program.
Next you will find the Office 2013 Language Preferences tool in the start menu, and this is the tool you use to configure languages for Office 2013.
Set the language you want to use in Lync 2013 spellcheck as the <default> language.
And look! It works!