How do I internationalize my CMS?

Background Denizen is a CMS for personal websites. I want to work on multi-language support – people should be able to make websites and posts in any language they want. (My own blog is in English, Turkish, and Toki Pona – so this is a blocker for eating my dog food). Every aspect of this feels like something other people must have already solved, so I wanted to publicly ask if anyone has the…