All this despite the fact that a significant chunk of the population speaks Russian as their native language, and some number of those people don't speak any other language. For example, in Daugvapils, a town that is 85% Russian speaking, I didn't see a single street sign in Russian. All around me, everyone was speaking Russian, but nothing was written in Russian, not even the advertisements or the shelf markings in the supermarket.
At one point, I was riding in a minibus back to Vilnius airport to pick up my backpack. I'd been waiting for the regular bus but it turns out that bus only runs once an hour. However, there was a woman waiting at the stop, and when a minibus showed up, she said, "aeroport?" I nodded, and we both got on the minibus.
As we rode along, I realized that the woman and the driver were speaking Russian, not Lithuanian. So when I got out of the minibus, I thanked them in Russian. Big smiles all around.
From then on I tried to pay close attention to what language someone was speaking before I said anything.