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.
Latvian language
According to some glottochronological speculations, the Eastern Baltic languages split from Western Baltic (or, perhaps, from the hypothetical proto-Baltic language) between 400 and 600. The differentiation between Lithuanian and Latvian started after 800, with a long period of being one language but different dialects. At a minimum, transitional dialects existed until the 14th century or 15th century, and perhaps as late as the 17th century.