I'm writing it in Elixir and Phoenix (Elixir's goto web framework) with PostgreSQL to back my data. For the front end, I'm using good old fashioned server side rendered templates and Turbolinks with sprinkles of Javascript where needed.
Despite having worked with Flask and Rails for the last 6-7 years I'm going for Elixir this time around because there are aspects of the app that lend itself well to what Phoenix has to offer and I'm using this as an opportunity to grow as a developer. Every time I learn a new language / framework I find myself leveling up in other technologies I know.
So far I'm super excited with the way things are turning out. I'm still pretty early on in the project but I've gotten a decent amount of functionality implemented already.
I'm really surprised Elixir isn't more popular. The language author and the supporting community is awesome, and I've been the happiest I've ever been working with this tech stack. Of course I'm still drinking the "omg this is new and awesome" kool-aid, but even without really knowing everything too well, I'm able to accomplish real world tasks and that's all I care about in the end.