I believe you will find the correct term is in fact "SLAVONIAN", pertaining to the language now spoken, in its various dialects, in Russia, Poland etc.
I seem to remember when we looked at this issue, it was also known as "SCLAVONIAN or SCLAVONIC".
The Slavs were of Eastern European origin and included many inc:- The Poles, Bulgarians, Serbs, Czechs, Russians and The Wends.
Now, here is where i'm not sure but the earliest use we found was in a Medievil Latin where it appears as Sclavus, but i'm sure in translation if this was also referring to the name of the language, as well as the people?
It is speculated that the name originated with The Wends, but we couldn't confirm this?Hope
this helps, nice to think we might be able to help you for a change..