I was also surprised to find out (roughly a year ago) that Claude is good at Old English (which, despite its misleading name, looks nothing like English and is more of a Germanic language) whereas ChatGPT would output pure hallucinations.
For example, when ChatGPT was outputting nonsense in Georgian, Claude was speaking it fluently, when ChatGPT learned Georgian, Claude was able to speak Mingrelian.
Your description of Old English is a bit odd. It's certainly very different from modern English, but it's its direct ancestor and both languages are Germanic.
[0]: https://www.poetryfoundation.org/poems/43521/beowulf-old-eng...
I've also tried Old Norse, Ancient Greek, and Old East Slavic, and the result is pretty much the same. For OES in particular, it often outputs period-incorrect grammar, writes in Old Church Slavonic (different language), or even modern Russian or Serbian. Looks like the dataset was a bit chaotic, with religious books mixed with old manuscripts and even modern books for children. Mentioning a specific work from the desired period makes it write better, and wrangling it by specifying the rules makes it get this almost right.