Language Classification for Posts

Let me walk you through how I'd design a language classification system for social media posts at massive scale. I'll go stage by stage — business and ML objectives, high-level design, data and feature strategy, embeddings, pipeline architecture including modeling and training details, infrastructure, evaluation, and robustness. This is one of those problems that sounds deceptively simple — "just figure out what language this post is in" — but it's actually a critical infrastructure component that needs to run on billions of posts daily with sub-millisecond latency and feed dozens of downstream systems. Getting it wrong cascades failures through the entire platform.