When it comes to western Europe, medieval Latin would be closer to an "official" language, especially for international affairs. The Roman Catholic Church's power and influence at the time was unparalleled and several major events of the era started with a Papal Bull. Here's a short list of Papal Bulls that were political in nature and were addressed, formally or informally, to the monarchs of the era:
- Sicut Judaeis (1120): Provides protection for the Jews who suffered from the hands of the participants in the First Crusade,
- Omne Datum Optimum (1139): Endorses the Knights Templar,
- Quantum praedecessores (1145): Calls for the Second Crusade,
- Laudabiliter (1155): Gives the English King Henry II lordship over Ireland,
- Manifestis Probatum (1179): Recognition of the kingdom of Portugal and Afonso Henriques as the first king,
- Audita tremendi (1187): Calls for the Third Crusade,
- Cum universi (1192): Defined the Scottish Church as immediately subject to the Holy See,
- Post Miserabile (1198): Calls for the Fourth Crusade.
Several other historical documents of the 12th century are written in medieval Latin, even if not issued by the Pope or the Roman Catholic Church. For example:
One document that stands out is the Charter of Liberties (1100), a forerunner to the Magna Carta. While apparently the original was most probably written in Latin, several copies of it were made in other languages depending on the intended audience, and all could have been considered authoritative at the time. The Magna Carta itself though was originally issued in Latin (and 15 years into the 13th century), and then translated to French.
In the east, however, Byzantine Greek had replaced Latin as the official language of the Byzantine Empire since 620, when Emperor Heraclius started styling himself as Βασιλεύς1 instead of the Latin Augustus. Official documents of the era were written in Byzantine Greek, although it's not unreasonable to assume that at least some of the documents that were addressed to European Monarchs would be written in medieval Latin.
Given that "official" at the time mostly meant sanctioned by the church, either the Roman Catholic Church or the Eastern Orthodox Church, I'd say that medieval Latin and Byzantine Greek would be very close to what we would today consider official languages.
1 King; sovereign.