The identity of the medieval Bosnian state is reflected in the language and alphabet used on its territory. Many lay and church writings, as well as burial and other inscriptions written for Bosnian rulers, lords and church dignitaries, tell us about the identity of the people they were written for.
On the territory of medieval Bosnia, some of the oldest Cyrillic written monuments in the South Slavic area were created, written in the folk language. Care should be taken to make the difference between the written monuments which originated from the original Bosnia and those monuments originating from the territories that later entered the state of Kotromanić. We will also pay attention to the research and the results of the scientists who dealt with the language and alphabet of the medieval Bosnian state.
Charters and letters
The oldest written monument in national language from the area of primary Bosnia is the letter of ban Kulin to Dubrovnik from 1189 written in Cyrillic.  Texts written in national language had the characteristics of the local dialect.  Texts of the Bosnian charters and other documents were written in uncial Cyrillic until the last quarter of the XIV century, not only in manuscript books, as it was the case within Nemanjići state, but also in diplomatic texts. 
Regardless of dialectical differences, in the charter of ban Kulin, as Vatroslav Jagić says, "such security and agility can be seen while writing in Serbian language with Cyrillic letters; such emancipation, greater than in the latter period, after the influence of the Church Slavonic, that it is not impossible to believe that in Bosnia, Zachlumia, Dioclea, etc. inhabitants started writing in Cyrillic and speaking in Serbian folk language long before ban Kulin".
Three years earlier, the ruler of Zachlumia, Prince Miroslav and his brother Stefan Nemanja, placed their Cyrillic signatures to a contract with the Dubrovnik people written in Latin.  This is the first written Cyrillic Serbian trace in the charters from Zachlumia. One year after Kulin's charter we have another signature by knyaz Miroslav, in another contract with Dubrovnik, again in a Latin, with his cross placed on the contract as well. 
In charter of ban Kulin and charters of other Bosnian rulers, created during the XIII century, there are similarities and commonalities in letter design with charters from Nemanjići dynasty in the same period, and based on this we can follow their common development line. For example, In the charter of ban Ninoslav from 1240, the letter "d (d)" is written with three moves: two divergent upright lines connected at the bottom by a horizontal line. The same is found in the transcript of the Stefan Nemanja’s charter from 1186.  Nemanja's scribe, as well as Ninoslav’s, writes the letter "ž (ž)" with three moves: two lines diagonally crossed, then crossed upright by a third. 
First to clearly identify language in Bosnian state is ban Stefan II Kotromanić in his charter to Dubrovnik from 1333. Four copies of the charter were made, two for Dubrovnik and two for Bosnian ban. Two copies meant for Dubrovnik were in Latin while the other two were in Serbian. Latin language was used back then in Dubrovnik, and as we can see, Serbian was used in medieval Bosnia. Charter was written in uncial Cyrillic, like all other domestic documents issued by ban.
Dubrovnik, and its chancellery for diplomatic correspondence with city’s hinterland, also sheds light on the issue of language identity of the medieval Bosnian state. Namely, since the beginning of the XIII century, entire official correspondence of Dubrovnik with states in the hinterland was conducted in Serbian language. Latin documents in Dubrovnik translate term Serb as Sclavus, while Serbian language is translated as sclavonico. Scribe of Serbian charters in Dubrovnik is translated in Latin as cancellarius sclav(ic)us, sclavonicus.
Thus, the part of ban Stefan II charter, which in Serbian version reads “two Serbian and two Latin”,  in the Latin version of the charter is translated as duo scripta in gramatica latina, et alia in sclauonico. One of the letters from ban Stefan II to Dubrovnik, from 1351, is preserved but only in its Latin version. Prior to text of the letter itself, there is a remark in Latin which, amongst other things, says “Ehemplum cuiusdam liters sclauonesche misse per dominum banum Bossine Communi Ragusii.... Translated to contemporary Serbian language, this reads as: “A transcript of a Slavic letter, sent by lord ban of Bosnia to Municipality of Dubrovnik.... In the second half of the XIV century there is an explicit mention of Niko Bijelić as Serbian dijak (scribe) in Dubrovnik. Mention dates from the year 1364, and four years later, while writting a document in Italian, Niko will say for himself: A mi Nico de Biele, scrivan scauonesco.
This clearly indicates one Serbian notary office existed in Dubrovnik during medieval period. It conducted official correspondence with surrounding Serbian states and lords, in Serbian language and Cyrillic alphabet, plainly indicating identity of those with whom they corresponded and negotiated merchant contracts.
Uncial Cyrillic was used in the time of ban Tvrtko as well, all up to the moment of his coronation for a king, after which cursive Cyrillic gains prevalence in the royal charters. Uncial Cyrillic was still used however, and some charters were written in it. In preparation for appropriation of tradition and throne of his parents and ancestors, the Nemanjići, Tvrtko started using Serbian cursive few years before he would be crowned a king, as we can see in an example of one letter to Dubrovnik.  Afterwards, Bosnian chancellery adopts the cursive Cyrillic, formed in Rascian chancellery during the reign of king Milutin Nemanjić. After its formation, cursive Cyrillic became general chancellery writing for documents written in Serbian language.
Significantly earlier, cursive writing was already present in Zachlumia which was part of the Nemanjići state when this typological variant of Cyrillic was formed in Rascia.
In medieval Bosnian state, if we exclude Croatian territories it would encompass periodically, Stokavian dialect was spoken in Bosnia, Usora, Soli, Donji Kraji (Lower Regions). In geographical terms, this meant the territory somewhat east from Una river all the way to the river of Drina. This area was dominated by ikavian speech, while in Zachlumia, ijekavian speech had prevalence.
From an ikavian speaker, Pribislav Pohvalić, herald of Sandalj Hranić and his wife Katarina, we have a surviving testimony that the ikavian speech was identified as Serbian language. Namely, Pribislav Pohvalić in his letter from 1407 identifies his ikavian speech as Serbian in this manner: with other Serbian page, written in Serbian; and in the Pribislav’s certification from the same date: “written here on this page in Serbian”. It goes on “in second page written in Serbian and more in the first page written in Serbian.” 
The same Pribislav, in his certification from 15th of March 1411 calls his language Serbian: these pages written in Serbian; while in another certification from the same date: this page written in Serbian, with another page written in Serbian. 
From the analyzed charters and letters created on the territory of medieval Bosnian state, written by Bosnian rulers and lords, we can see they are written in Serbian language and in Cyrillic of Serbian redaction.
Author: Boris Radaković
 F. Miklošić, Monumenta Serbica, Vienna, 1858, 1-2.
 B. Čigoja, Following traces of Serbian Language Past, Belgrade 2006, 17.
 P. Đorđić, The History of Serbian Cyrillic, Belgrade, 1990, 129-130.
 V. Jagić, History of Literature of the Croatian and Serbian People, vol. I, Ancient age, Tome 1, Zagreb 1867, 142.
 P. Đorđić, the aforementioned work, 65.
 Same, 65.
 Same, 92.
 Same, 92.
 F. Miklošić, Monumenta Serbica, Vienna, 1858, 107.
 L. M. Kostić, Violent Acquisition of Dubrovnik Culture, Melbourne 1975, 41.
 M. Dinić, Serbian lands in the Middle Ages, Belgrade 1978, 34.
 D. Ječmenica, Ston Charter of Stefan II Kotromanić, Materials on history of Bosnia 3, Banja Luka 2010, 39.
 M. Brković, Latin charter of Bosnian ban Stjepan II Kotromanić from the year 1333 (15.II), Historical Review, 10, Banja Luka 1989, 9.
 D. Ječmenica, Letter from ban Stefan II inviting Dubrovnik merchants to freely trade in Bosnia, Materials on history of Bosnia, 5, Banja Luka, 2012, 28-29.
 R. M. Grujić, Apology of the Serbian people in Croatia and Slavonia, Novi Sad 1909, 34-35.
 D. Ječmenica, Two Letters from ban and King Tvrtko I to Dubrovnik about Ston tax, Materials on history of Bosnia, 1, Banja Luka 2008, 41.
 P. Đorđić, stated work, 145.
 Same, 145.
 Sima Ćirković, History of Yugoslavia, Belgrade 1973, 138.
 Lj. Stojanović, Old Serbian Charter and Letters, Book I - Part One, Belgrade-Sr. Karlovci 1929,337.
 Same, 338-339.
 Same, 346.
 Same, 348.