This is a pretty frequent thing to encounter actually. Just some years ago many websites actually preferred to use ISO/Windows codepages to save space on multibyte Unicode symbols adding HTML entities to represent everything which is not in it the basic ASCII and their primary language alphabet.

Fun fact: I was looking for an e-mail solution for a small company about a decade ago and found Zarafa. It seemed nice and I deployed it happily. Just to find out it only supports the Western European ISO codepage which was hardcoded. I hope they have switched to UTF-8 since then.