The Balkans were colonized with Latin speakers by the Roman Empire. In particular, there are many hints about Romanian language's origin in the Vulgar Latin dialects of Northern Italy, as it has some parallels with the modern dialects of the Veneto region (words or phonetic changes that are not found in other Romance languages). Of course, there were immigrants from all the corners of the Empire, including the Levant, as well as locals who were assimilated.

Romanian language is a very unified language, there are few differences between dialects, so this means that the spread is of recent date, after the language was already formed. Even the Aromanian language from Greece is still relatively close in grammar and core vocabulary, despite being harder to understand to Romanians because it had a large amount of Greek loanwords instead of Slavic ones like Romanian.

Now the question is where was the place where this language formed itself. This is still a very contentious issue, as historical sources are scarce and archeology cannot tell what language a population spoke. It's obvious that Romanians lived in the northern part of the Balkan peninsula, as the southern part was Greek-speaking. Romanian nationalists insist that Romanians lived in a region of today's Romania, but there are some indications that this was further south.

For instance, the Aromanian word for fig, "hică" (cf. higo in Spanish, fico in Italian) is inherited from Latin, whereas Romanian lost its word, borrowing it later from Slavic ("smochină"). This indicates that Proto-Romanian (from which both Romanian and Aromanian derived) was spoken further south, in a place where figs grew and as Romanians moved northwards, they lost the word, whereas Aromanians always lived in fig-growing places.

Today's territory was at one point almost completely Slavic speaking (there is no county without dozens of placenames of Slavic origin, if not hundreds), but it was very low density, which made it easy for Romanians to assimilate them.