Looking at https://banana.dog/packs/js/common-34b349e09081ef8d0772.js makes me wonder: why can't we extract the locale information into a JSON file and cache it with a service worker?
After all, this pack is quite literally the largest bit of JS. It's around 1MB. I might contribute this knowledge to Mastodon...
In terms of JavaScript, Mastodon's UI is quite light aside from this.