John:
You are right, słowiańska means just Slavic in Polish, and yes, Thietmar just mentions her as the daughter of "Dobremirus" and does not give any information about where she is from. I gather that the origin from Lusatia is guessed at mostly based on onomastic reasons: both Emnilda and her daughter's name Regelindis appear in Saxon nobility hence the speculation that Dobromir was married to a Saxon and Lusatia would be the most logical Slavic area of this time bordering Saxon lands.