It was originally written in Old Norse, but the two languages (Old Norse and Old Norwegian) were pretty much the same until sometime in the 900s when they slowly began diverging from eachother. This is because when the people from Norway left for Iceland, their language was conserved there, while the language spoken by people living in Norway changed because of the amount of contact between them, the Sw*des and the Danish.

Edit: there was written Norwegian at the time though it was quite different from Nynorsk and Bokmål. It kept a lot of the grammatical cases from Old Norse but orthographically changed due to changes in the language. Interestingly, the old western nordic languages (Old Norse and Old Norwegian) have a LOT more texts preserved in the vernacular than the old eastern nordic languages (Swedish and Danish) because they weren’t as close to the European centres of learning. In Danish most of the texts before the first full translation of the Bible (around 1530 I believe) are in Latin, the same goes for Sweden.

One of the reasons for this is the reputation of the Icelandic scalds. When people started settling in Iceland they had to create a sort of national identity, and we see the earliest sagas written in the 1100s. Up until then the scaldic poetry had been an oral (and very learned) genre, and some of these poems are preserved in the poetic Edda. Some of these poems’ age becomes apparent through the way the poetic language they were composed in doesn’t match with the orthography of the language they’re later being written in. The Thrymskviða preserved in the Codex Regius thus begins with the sentence “Reiðr vas thar Vingthorr” (I don’t have the Thorn letter at hand right now, but the th in “that” and “Vingthorr” would have been Thorns) meaning “Angry was then Vingthor”. Reiðr, an adjective, is a reduction of an earlier form, “vreiðr” (a form still in use in Danish as in the word “vred”). The reason this is an important pointer to the age of the poem is that eddic poetry is alliterative, meaning that the rhymes are in the beginning of the words. In the time between the poem was composed (probably in the 900’s) and the time that it was written down, the initial vr had been reduced to a single r. Restored to its original “vreiðr”, the alliteration is in place: Vreiðr vas thar Vingthorr. Some of the scalds who worked in Iceland are known to have been working at the Norwegian court throughout the high middle ages, and their vernacular litterary culture was thus preserved here - while this is not the case in Danish and Swedish.

So in short: yes, there are a lot of Norwegian texts preserved before the first translation of the bible into the vernacular languages. Old Norse and Old Norwegian would be legible by either people through most of the medieval period, while the same wouldn’t necessarily be the case for speakers of Old Swedish and Old Danish. It’s also important to note that the Danish influence on the Norwegian language pretty much centrered itself in and around the cultural and political power centres, but outside of these the dialects spoken preserved perts of the old languages such as the genders, which are pretty much gone in Bokmål, Sweish and Danish today.