Latgalian is a language similar to Latvian, the official one. Unlike the rest of Latvia, where only the cities are heavily russified during the soviet period, in Latgale even many countryside areas are more Russian than Latvian/Latgalian-speaking.
Latgale has hundreds of lakes. Make sure to swim in some of them in summer or to see them at least. Latgale is also rather hilly, even though the hills are not high compared to other countries. Latgale is also famous for its pottery. Visit some pottery workshops and countryside houses. A boat trip in lakes is a good idea.
Try the traditional pork products. Delicious!