Latgalian language
Originally Latgalians were a tribe living in modern Vidzeme and Latgale. It is thought that they spoke the Latvian language, which later spread through the rest of modern Latvia, absorbing features of the Old Curonian, Semigallian, Selonian and Livonian languages. The Latgale area became politically separated during the Polish–Swedish wars, remaining part of the Polish–Lithuanian Commonwealth as the Inflanty Voivodeship, while the rest of Latvians lived in lands dominated by Baltic German nobility. Both centuries of separate development and the influence of different prestige languages likely contributed to the development of modern Latgalian as distinct from the language spoken in Vidzeme and other parts of Latvia.
Latvia
Several important cultural changes occurred during this time. Under Swedish and largely German rule, western Latvia adopted Lutheranism as its main religion. The ancient tribes of the Couronians, Semigallians, Selonians, Livs, and northern Latgallians assimilated to form the Latvian people, speaking one Latvian language. Throughout all the centuries, however, an actual Latvian state had not been established, so the borders and definitions of who exactly fell within that group are largely subjective. Meanwhile, largely isolated from the rest of Latvia, southern Latgallians adopted Catholicism under Polish/Jesuit influence. The native dialect remained distinct, although it acquired many Polish and Russian loanwords.