Vilnius 1940

This is a registered cover sent in 1940 from Vilnius 4 during the Soviet occupation period to the Swiss capital Bern. It has a nice arrival cancel of Bern on the back. It even has a letter inside still but unfortunately my Polish sucks therefore I'm unable to tell you guys what is it written about... Given the fact that the Soviet took Vilnius by force from the Poles, I can imagine that the Polish author of this letter (it's obvious from his Polish name and the Polish text) didn't just write a "what's up dude"-kinda letter but rather a serious one. The interesting part is that he wrote his address as "Vilnius, Latvia", which makes it look like he probably isn't from Vilnius/Wilno originally. It is indeed pretty confusing for those who is not familiar with Lithuania's name in other languages, Most worldwide stamp collectors probably know that Lietuva is the Lithuanian name for Lithuania as you can see on their postage stamps but it sounds and looks closer to Latvia than Lithuania. I actually don't know why is it called Lithuania in English...