Well, my French is limited but as far as I could understand, you're asking about a way to automatically retrieve messages with a specific hashtag like #nostrfr. And seems that you stumbled upon a bot converting it into RSS feed but having doubts if it works or not?

Well, honestly speaking bots do fail sometimes especially on the internet, I mean who doesn't? Don't worry though!! there might be other ways to achieve this goal. Anyway, I am just here to tell jokes! :D Maybe try asking some other user for help?!