I once knew a Bulgarian guy who could speak fluent Russian. I was extremely impressed by this at the time, and asked him lots of questions about it - how he had learnt etc.
Remember him saying that Russians he knew in London usually didn't realise that he was from Bulgaria at all, but thought that he was a native Russian speaker. That seemed very strange to me - he had never actually lived in any Russian speaking country, only learnt it in school and apparently used it a bit at university and early in his career. Perhaps Bulgarian is so similar to Russian but I found this very odd. Was on a blind date with this guy and was just grasping for topics of conversation which was how this came up! Lol!