> is a bit of a junior- to mid-level engineer tell in my eyes

There is absolutely no reason for Discord—a chat application—to run everything a browser does. Why do I need to play videos in the chat application? Why do individual avatars and even tags have to run custom shaders? It's called scope creep.

> a11y, IME, RTL

All of this can be done at the platform level. Win32 did this three decades ago. Comes with entire language packs.

> so you add a scripting layer, maybe integrate a little Lua VM or something that lets you execute JavaScript

See above: scope creep; features for features' sake.