Google Translate does not offer Old Norse. This translator asks Google for Icelandic, the closest language it does offer, and then rewrites that into Old Norse with a documented set of rules: word swaps, sound changes and grammar. It is a rule-based rewrite, not a human translation, so check anything that matters with a speaker.
40.0 on 163 real sentences; Google's Icelandic as is: 36.5.
The score is chrF: how closely the output matches what real Old Norse speakers wrote, character by character, from 0 to 100. Tested on 163 Old Norse sentences from Wiktionary with their English, held out and never read: the English goes through the course's own route (Google into Icelandic, then the mod) and is scored against the real sentence; no second translation exists to give a human ceiling.
Modern Icelandic rewritten into Old Norse by rules: the classical forms of the common words (ok, at, þat, svá, váru, hon, mjök, mik), the -r of maðr and kemr where Icelandic added a u, the -t of mikit and kallat, -lig- for -leg-, and vér and þér for the modern plurals. The words are learned from saga editions printed in both spellings. No output is written by an LLM.
The rules are a plain file anyone can read, test and copy. Every translation shows Google's Icelandic underneath, so you can see exactly what the rules changed.
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