The roman empire had two main languages, greek and latin. But while latin gave birth to french, portuguese, spanish and italian, greek language is now only spoken in Greece. Why the germanic tribes adopted latin but the slavic tribes did not ?
Because the West was destroyed and the tribes squatted on its remains, using its legal system, church structure etc.
Slavs destroyed all that they found in their ravages of Pannonia, Thrace, Dalmatia etc. in the 6th and 7th centuries and did not take to Christianity (while the tribes in the west mostly had already been converted while they were outside of the Roman Empire, the Slavs had been too far outside to be converted).
Slavs essentially didn't bother to keep hold of any civilisation that they found since they did not associate Romanitas with their religion. It took many centuries for them to get anywhere near to where the Visigoths, Franks etc. got. Even the illiterate Saxons, Frisians, Jutes and Angles managed to convert quicker and then create unified kingdoms.
Fun Fact: Syria, the Levant, and Egypt spoke Greek.
Well, only the Ruling Class & Literati desu.
Also Greek is the diplomatic lingo in Middle East. The Sassanids talked to Romans in Greek for example. It was the case even early into the Caliphate until Arabic completely took over.
What are you talking about? Of course Slavs converted to Christianity. It didn't happen instantaneously but it started between the 7th and 8th century and conversions started in Kievan Rus in the 10th century, mostly because of the distance.
>Slavs essentially didn't bother to keep hold of any civilisation that they found
Greek actually sounds nice, especially when compared to how the language of their eastern neighbours sounds
Greek encroached on the largest urban civilizations of human history, Latin encroached on mudhut barbars - yes, they had cities, but a highly primitive and sparsely-populated civilization compared to the East.
You'd probably have seen Egypt and the Levant speak Greek en masse if the ERE lasted longer, but Greek was competing with cultural capital close to its own in Syria and Egypt.
As for the northern Frontier, in the Balkans Latin was actually the dominant Roman tongue north of a certain point, even in the eastern Balkans, and you saw even Latin lands in the West being submerged by filthy barbars along the borders (see: trans-rhine Germany and Flanders, German Switzerland and Austria). Same thing happened in the East.