Baltic laguages? ... Interesting languages ... all are very different and on the other side very near to Sanskrit. Astonishing. And I see the Prussians were also a baltic population once. When and why did the Prussians start to speak German?
Most of them didn't "start" to speak German. Most of them were genocided by Teutonic Order in XIII century.
I don't see a big sense to call the own people "glorious". Normally everyone calls the own people something like "old idiots". On the other side think lots of people often the own criminals are saints and the saints of foreign nations are criminals. In case of the word "Slaw" I guess "word" is the root. Perhaps something like "people, who got the word"
Who knows? There was a period, when they called themselves as "Soviets" (from "soviet" - "advice", "council"). May be, it was a self-identification based on the linguistic sign: "We can speak the language, we are not speechless barbarians", may be, it was based on lifestyle: "We are reputable men, we are not unknown strangers", may be, it was based on their political system "We are following glorious leaders, we are not subjects of hereditary monarches".