This is the 3569th most frequent Polish word.
bułgarii
"Bułgarii" is the locative or genitive form of "Bułgaria," which means "Bulgaria" in English.
Here, 'Bułgarii' is used in the locative case to indicate the place where the action occurred.
Byłem w Bułgarii latem.
I was in Bulgaria in the summer.
In this sentence, 'Bułgarii' is in the genitive case, showing possession or relation.
Znam historię Bułgarii.
I know the history of Bulgaria.
The word 'Bułgarii' is in the dative case here, used after the preposition 'do' to denote direction or target.