You are right. During Soviet times in all the territory there was one general Russian taught. Many small languages didn’t survive.
Deniss’ first lg in family is Russian and he certainly speaks it well. Yea, there might be a slight accent (he doesn’t palatalize sounds so much as Russians do) but it is because from the age of 9 he has lived so much outside Latvia and had to learn French and English. Maybe he has some Latvian roots as well (it’s only my guess) because he hasn’t got the typical Russian accent most of them have when speaking Latvian.
Deniss considers himself a Latvian. Still, he sometimes finds it difficult to express himself in Latvian but he always tries. That’s the reason why he often switches to English- then it goes quicker. It’s understandable as he actually doesn’t have any practice. But he has a good tongue for languages. If he lived 2-3 weeks in Latvia among native speakers, he would speak fluently.