What those instruments have common? In which country those instruments are used in traditional music? Yes, this is Croatia, but We mainly associated Mandolin with italian music, specifically with music from Naples.
Croatian traditional music is sometimes divided on two parts. Slavonian tradition and Dalmatian tradition, but there are also regions, which are not parts of Slavonia or Dalmatia. By those things, We can divided croatian musical tradition to coastal and inland. There are also lot of another styles, but I will describe mainly those, where are Tamburica and Mandolin.
Tamburica is popular in Slavonia and inland regions of Croatia. Tamburica is also popular in neighboring Serbia, specially in region Vojvodina. This region was influenced by historical Hungary and music countains some central european influences or same, like in Vojvodina. Tamburica is also native instrument of Croatia.
Dalmatia was controlled by Veneto, so theyr music is different. There is also some similarities between Klapa and Friulian music too. Klapa is something, like a cappella, but there are sometimes used musical instruments, like guitar, or mandolin. Nowadays, Klapa countains modern music characteristics. I think, that some people, which were in holiday in Croatia, maybe heard those kind of music.