In the 8th and 9th centuries, the first vowel in the word svat ("holy") would have been a nasal front vowel, probably pronounced something like the sound in the French word vin, which is why the Franks spelled it -en-. It evolved differently in different Slavic languages, turning into something we usually transliterate as "ja" or "ya" in Russian (e.g. �?в�?той = svjatoj, "holy" – though it's more complicated than that), an a or á in Czech (compare the names Václav and Wenceslaus), the nasal sound iȩ in Polish (święty means "holy"), and a high front e in the South Slavic languages (svet means "holy" in both Slovene and Serbo-Bosno-Croatian).
The second part of the name would in Old Slavic have originally been pronounced more or less pulk (rhyming with the English word hulk), where the "u" stands for a short schwa sound. This meant something like "host" or "army" ("a mass of people"). It is actually a borrowing from a Germanic root that has developed today into folk in English and Volk in German (which makes the Franks' hearing it as a different Germanic word somewhat ironic). This word also developed differently in the different branches of Slavic: polk in East Slavic (e.g. Russian) and South Slavic (e.g. Slovene), and pluk in West Slavic (e.g. Czech).