A note on the Croatian letter 'Lj'. This is considered to be one letter in the Croatian alphabet, and is pronounced like the double L sound in 'million'. (Remember: the J is pronounced like a Y.)
Ljestvica means 'musical scale', and Istarska ljestvica refers to the musical scale unique to Istria. It's characterized by two-part singing which involves two singers singing together, but an octave apart, and using a technique of 'partial' nasal singing, ending with the two singers singing in unison. The second voice can be replaced by an instrument like sopele (a traditional woodwind instrument) or bagpipes.
This is difficult to describe in words, but this short video provides a useful demonstration:
This unique tradition of two-part singing and playing in the Istrian scale is recognized by UNESCO and was inscribed on its Representative List of the Intangible Cultural Heritage of Humanity in 2009.