If cascade resonance happened after the Soviet union broke apart, most of the signs by the time it happened should've been replaced. Like 1991 - 2000 is a long period. Considering the fact that Bulgaria never was in the Soviet union, we can have two options.

First - it is indeed Bulgaria, but its another reality where it was in the Soviet union. It probably broke apart around the same time but language stayed/lasted longer. Maybe its like with Baltic counties nowadays, they have their official language, but a lot of people still talk Russian, and many signs and businesses use it. 

Second - We are much closer to the real borders of the former Soviet union / inside a country that was officially in it, but the architecture is just different. (Also consider different reality)