Rachel it is unclear what language as it is not part of the Rohan songwriter's song, but something stated by the narrator as part of a list of Gondorians other than Halbarad who died. The list included Forlong, Duilin, Hiluin, Grimbold plus Halbarad. Thinking again of Exploring LotR this sounds very much like an insert of Findegil Kings writer.
I paste the paragraph below:
Aragorn and Éomer and Imrahil rode back towards the Gate of the City, and they were now weary beyond joy or sorrow. These three were unscathed, for such was their fortune and the skill and might of their arms, and few indeed had dared to abide them or look on their faces in the hour of their wrath. But many others were hurt or maimed or dead upon the field. The axes hewed Forlong as he fought alone and unhorsed; and both Duilin of Morthond and his brother were trampled to death when they assailed the mûmakil, leading their bowmen close to shoot at the eyes of the monsters. Neither Hirluin the fair would return to Pinnath Gelin, nor Grimbold to Grimslade, nor Halbarad to the Northlands, dour-handed Ranger. No few had fallen, renowned or nameless, captain or soldier; for it was a great battle and the full count of it no tale has told. So long afterward a maker in Rohan said in his song of the Mounds of Mundburg: