Many characteristics of the Dacian language are disputed or unknown. The only extant documents in Dacian are a handful of short inscriptions. What is known about the language derives from:
Dacian used to be one of the major languages of South-Eastern Europe, spoken from what is now Eastern Hungary to the Black Sea shore. Based on archaeological findings, the origins of the Dacian culture are believed to lie in Moldavia, being identified as an evolution of the Iron Age Basarabi culture.
In the 1950s the Bulgarian linguist Vladimir Georgiev published a work in which he argued that the phonology of Dacian is close to that of Albanian, supporting the theory that Dacian was on the same language branch as the Albanian language, a language branch termed Daco-Moesian (or Daco-Mysian) — Moesian (or Mysian) being thought of as a transitional dialect between Dacian and Thracian.
There are cognates between Daco-Thracian and Albanian. These cognates may be evidence of a Daco-Thracian-Albanian language affinity.
The ancient Greek geographer Strabo claimed that the Getae spoke the same language as the Thracians. However, Georgiev argued that Dacian and Thracian are two different languages, with two different phonetic systems, supporting this view with the evidence of placenames, which end in -dava in Dacian and Moesian, as opposed to -para in Thracian placenames. (See List of Dacian cities and List of ancient Thracian cities.)
It is unclear exactly when the Dacian language became extinct, or even whether it has a living descendant. The initial Roman conquest of part of Dacia did not put an end to the language, as Free Dacian tribes such as the Carpi may have continued to speak Dacian in the area northeast of the Carpathians (in the areas of modern Moldova and Ukraine) as late as the 6th or 7th century AD.
The argument for this early split (before 300 BC) is the following: Inherited Albanian words (e.g. Alb motër 'sister' < Late IE ma:ter 'mother') show the transformation Late IE /a:/ > Alb /o/, but all the Latin loans in Albanian having an /a:/ show Latin a: > Alb a. This indicates that the transformation PAlb /a:/ > PAlb /o/ happened and ended before the Roman arrival in the Balkans.
On the other hand, Romanian substratum words shared with Albanian show a Romanian /a/ that corresponds to an Albanian /o/ when the source of both sounds is an original Common /a:/ (mazãre / modhull < *ma:dzula 'pea', raţã / rosë < *ra:tya: 'duck'), indicating that when these words had the same Common form in Pre-Romanian and Proto-Albanian the transformation PAlb /a:/ > PAlb /o/ had not yet begun.
The correlation between these two facts indicates that the split between the Pre-Roman Dacians (those Dacians who were later Romanized) and Proto-Albanian happened before the Roman arrival in the Balkans.
The Dacian language may form the substratum of the Proto-Romanian language, which developed from the Vulgar Latin spoken in the Balkans north of the Jirecek line, which roughly divides Latin influence from Greek influence.
Whether Dacian in fact forms the substratum of Proto-Romanian is disputed (see Origin of the Romanians), yet this theory does not rely on the Romanization having occurred in Dacia, as Dacian was also spoken in Moesia, and as far south as northern Dardania. About 300 words in Eastern Romance (Romanian, Aromanian, Megleno-Romanian, Istro-Romanian) may derive from Dacian, and many of these show a satem-reflex, as one would expect in Daco-Thracian words.
The Romanian philologist Nicolae Densuşianu argued in his book Dacia Preistorică that Latin and Dacian were the same language or mutually intelligible dialects. His work was disregarded by mainstream linguists as pseudoscience, but it was revived by the Nicolae Ceauşescu regime, which encouraged an ideology called Protochronism and stressed the important role of the Dacians in the creation of the modern Romanian people.
The first article to revive Densuşianu's theory was an unsigned article named "The Beginnings of the History of the Romanian People" published in Anale de istorie, a journal published by the Romanian Communist Party's "Institute of History of the Party".
The article claims that the Thracian language was a pre-Romance or Latin language using a demonstration which Lucian Boia describes as "a lack of basic professionalism and a straightforward contempt for the truth". Arguments used in the article include the lack of interpreters between the Dacians and the Romans, as depicted on the bas-reliefs of Trajan's column. The bibliography includes, apart from Densuşianu, the work of a French academician Louis Armand (who is in fact an engineer), who allegedly showed that "the Thraco-Dacians spoke a pre-Romance language". Similar arguments are found in Iosif Constantin Drăgan's We, the Thracians (1976).
This generated a great interest on researching of history of Dacia and many (often non-rigorous) works were published, among them Ion Horaţiu Crişan's "Burebista and His Age" (1975), who concluded the need of writing a monograph on the subject of "Dacian philosophy". There were voices claiming the need of reconstructing the language and of the creation of a Dacian Language department at the University of Bucharest, but such proposals failed because of the lack of the object of study.
After the 1989 Romanian Revolution, this theory continued being supported by Drăgan and the New York City-based physician Napoleon Săvescu, who published a book named We are not Rome's Descendents. Together, they issue the magazine Noi, Dacii ("Us Dacians") and organize a yearly "International Congress of Dacology".