Short answers, pulled from the story.
Old Frisian is a West Germanic language classified as Ingvaeonic, a subfamily that also includes Old English and Old Saxon. It shares a common ancestor with these languages and with Low German, but the once-popular view that English and Frisian had a uniquely close genetic bond is no longer the prevailing scholarly position.
Old Frisian was spoken from the late 13th century to the end of the 16th century along the North Sea coast from roughly modern-day Bruges in Belgium to the Weser River in northwestern Germany. By the late 16th century it had evolved into Middle Frisian as a rural vernacular or been displaced by Dutch and Low German.
The surviving corpus is dominated by legal documents. All but one of the Frisian-language documents east of the Lauwers River are legal in nature, and most texts were compiled into seventeen legal codices. West of the Lauwers, over a thousand charters survive alongside poetry, historiographies, and religious works. The earliest full manuscripts are the First Brokmer Codex and the First Riustring Codex, both dated to between 1276 and 1300.
Scholars including De Haan and the British Old English specialist Alistair Campbell argued that Old Frisian was contemporary not with Old English or Old Saxon but with Middle English, Middle Dutch, and Middle High German. Rolf Bremmer traced the Old label to academic prestige, describing the naming system as functionally arbitrary, though he accepted the two Riustring Codices, dated to around 1300 and 1327, as genuinely Old by criteria rooted in Jacob Grimm's work.
Middle Low German began displacing Old Frisian as the language of North Sea trade by the end of the 15th century. The Duchy of Saxony's subjugation of Frisia following an internal conflict then imposed Dutch as the administrative language in the 16th century. The last known public document in Old Frisian dates to 1547, and by 1550 the language is considered to have evolved into Middle Frisian.
Old Frisian borrowed from Celtic languages, Slavic languages via Low German, Old French, Old Saxon, Middle Low German, Old High German, and heavily from Latin following the Christianization of the Frisians around the 8th century. Latin calques were especially common, including godeshūs for church and elemechtich for almighty, both modeled on their Latin equivalents.