Does anyone have a recommendation for a i18n file formats that are machine read/write-able? I'm especially interested in i18n formats that can work well with syntax extensions (like adding links). I have been considering Fluent since it has a Rust parser and AST tooling, but I haven't found a crowd-sourcing localizations editor compatible with Fluent, yet. (Hand-rolling that platform is on the table for sure, but I wanted to see if anyone else has thoughts)

The Phoenix web framework uses .po files. I’m not really familiar with them, but seems that are popular. I found https://poedit.net with a quick search, it looks cool

Awesome, Poedit has came up in my searches over a few years, but I didn't know that it was the choice for the Phoenix framework—that's a pretty good endorsement.