On Literature.SE we have language tags for non-English literarture. However, I'm not sure about the language tag we should use for troubadour literature. According to Wikipedia, troubadour poetry is written in Old Occitan. But the language of troubadours is quite particular and has also received other names. For instance, Martín de Riquer in his book Los trovadores. Historia literaria y textos uses provenzal (Spanish), which is usually translated as Provençal. According to this autor
Integran el conjunto de la lírica que llamamos trovadoresca las 2542 composiciones de unos trescientos cincuenta poetas de nombre conocido, y varios accidentalmente anónimos, que reciben el nombre de trobadors (caso sujeto singular trobaire), escritas en una lengua románica que comúnmente conocemos con el nombre de provenzal, que, aunque impropio, lo admiten la mayoría de los romanistas y es el que se ha divulgado entre la gente cultivada.
In English, this would be more or less
The set of lyrics we call troubadour poetry encompasses 2542 compositions by around three hundred and fifty poets with known names, as well as several accidentally anonymous ones, collectively referred to as trobadors (singular subject case trobaire). These compositions were written in a Romance language commonly known as Provençal, which, although somewhat inappropriate, is accepted by most Romance scholars and has become widespread among the cultured people.
Lucia Lazzerini (Università degli Studi di Firenze), in her book Letteratura medievale in lingua d'oc (in Italian) uses, indeed, lingua d'oc, which, according to Wikipedia, should be lenga d'òc in Occitan and langue d'oc in French, it's llengua d'oc in Catalan and lengua d'oc in Spanish.
The language of troubadours is in fact a literary koinē. In words of Riquer in the above cited book:
Al propio tiempo, la lengua de los trovadores ofrece una curiosa y evidente homogeneidad, que se impone por encima de las variantes dialectales de la zona lingüística de que procede cada poeta y que contrasta con los rasgos peculiares que se advierten en la épica y en la prosa y que han perdurado en la lengua hablada actualmente. La lengua de la poesía lírica es, pues, una especie de koiné que, teniendo tal vez como base última la variedad lingüística de Tolosa, adquirió la suficiente fiexibilidad para poderse cantar ante auditorios de localidades muy distantes sin que llamaran la atención giros ni fenómenos peculiares de un lugar determinado. Siendo fundamentalmente una poesía que era llevada de corte en corte y de ciudad en ciudad, tenía que amoldarse a un patrón lingüístico unificado, que para muchos poetas representaba cierta arbitrariedad y un consciente abandono de formas propias de su dialecto materno. Conviene advertir que este lenguaje homogéneo acepta numerosas alternativas en sus formas gramaticales, lo que da cierta libertad al poeta y hace que hoy día podamos sorprender soluciones fonéticas diversas, a veces en la obra de un mismo trovador.
In English, this would be:
At the same time, the language of the troubadours exhibits a curious and evident homogeneity that prevails over the dialectal variations of the linguistic region from which each poet originates. This contrast with the distinct characteristics found in epic poetry and prose, which have persisted in contemporary spoken language. The language of lyric poetry, therefore, is a kind of koinē, possibly rooted in the linguistic variety of Toulouse, and it acquired enough flexibility to be sung before audiences in distant locations without drawing attention to specific regional expressions or phenomena. Since it was primarily a poetry carried from court to court and from city to city, it had to conform to a unified linguistic pattern, which, for many poets, meant a certain arbitrariness and a conscious abandonment of forms specific to their native dialect. It is worth noting that this homogeneous language allows for numerous grammatical variations, providing the poet with a degree of freedom, which is why we can discover various phonetic solutions, sometimes within the work of the same troubadour.
To have some more information, it's also interesting to read what it's explained in the book The World of the Troubadours by Linda M. Paterson:
The identity of Occitania lies not in its political boundaries but in its language and culture.Occitan is a Romance or neo-Latin language, like French, Catalan, Franco-Provençal, Castilian, Portuguese, Italian, Sardinian, Romanian, Rheto-Romansh and Dalmatian. Its geographical extent has not changed significantly since the Middle Ages, though the number of native speakers has diminished dramatically. At the Pyrenees the Occitan language borders on Basque to the south-west, Castilian to the south and Catalan to the south-east. Its northern linguistic frontier starts at the confluence of the Garonne and the Dordogne and follows the course of the Gironde northwards to include the Limousin and part of Marche and the Auvergne, cutting the Rhône above Valence and passing south of Grenoble to join the Italian frontier. From here, with the exception of enclaves of Occitan dialects in the Piedmontese Val d'Aosta and Val di Susa, it coincides with the political frontier down to the Mediterranean. To the north-east, the Occitan frontier excludes part of the Dauphiné, which together with Savoy, western Switzerland and Franche-comté makes up the linguistic area of Franco-Provençal. An area of interference between Occitan and French, known as the crescent, includes parts of the Angoumois, Poitou, the Limousin, Berry, Marche, Auvergne and Bourbonnais. The Occitan linguistic domain therefore occupies, with a few additions at the margins, approximately a third of present-day France.
In the Middle Ages the Occitan language was defined in opposition to other languages. From the twelfth century the troubadours sometimes referred to it as lengua romana as opposed to Latin. In the thirteenth and fourteenth centuries some municipal documents named it roman by contrast with the langue du roi, and literary texts opposed it to frances. The Catalan grammarian Raimon Vidal de Besalú called it lemosi, as opposed to parladura francesca, while the term proensal or proensales seems to have been preferred by Italians for whom southern Gaul was still the Provincia romana, whose inhabitants, the Provinciales, were differentiated from the Francigenae of northern France. Dante seems to have been the first to use the expression lingua d'oco, opposing it to the languages of oïl and si ('yes' in Occitan, French and Italian). The term occitan was used by the official administration in the fourteenth century, in such terms as lingua occitana, respublica occitana, patria linguae occitanae and Occitania. In southern France today, the term Occitan is not without political charge. But medievalists prefer it because it avoids the ambiguity of an epithet linked to any one small part of the wider whole.
Linguistically, the troubadours perceive as foreign Breton, English, German, Frisian, Flemish, Angevin, Bavarian, Greek, Scottish and Welsh. Aside from Angevin, the speech of northern France, Spain and Italy provokes no explicit comments on difficulties. Some Catalan and Occitan authors could certainly understand French, and the poet of Girart de Roussillon tried to create a fusion of French and Occitan. Frequent exchanges between troubadours in Occitania, Italy and Spain created a certain internationalism, an interest and ability in foreign languages, reflected for example in their multi-lingual poems, which was not shared by the less open-minded French. Within Occitania, local differences must have been considerable: for example the fourteenth-century inhabitants of the Pyrenean village of Montaillou were conscious of a local dialect spoken by a thousand people at most. However, dialectal differences within Occitania appear to have played little or no role within troubadour culture, the poets establishing from the outset a literary language or koinē.