the idea that Marco Polo was Croatian is based on a misunderstanding that arose in Korcula (he was captured in the battle that took place near Korcula between Venice and Genoa)...it is played up by the local tourism industry but seems like harmless nonsense. There are some scholars who think that the Croatian name may be originally Iranian (there was an ancient Iranian tribe with a similar name)--possibly an Iranian elite conquered a group of Slavs and left them their name, the way that the Bulgars did to a different group of Slavs (the original Bulgars were not Slavic, but not Iranian either).
The people of medieval Ragusa were Catholics and spoke Croatian or a local Romance language which later died out (known as Dalmatian). A lot of people in the Balkans didn't have a firm ethnic identity until the 19th or 20th century--would have spoken a particular language and had a particular religion, but if illiterate peasants may not have affiliated themselves with one of the modern names.