YouDub-WebUI is a full-stack web application for video localization (dubbing and subtitling). It provides a user-friendly interface to download videos from YouTube, Bilibili, or local files, then runs a multi-stage ML pipeline: source separation (Demucs), speech-to-text (Whisper), translation via OpenAI Chat Completions, text-to-speech (VoxCPM2), and final audio/video merging with FFmpeg. The backend is a FastAPI server with SQLite persistence, task queue, and background worker; the frontend is a Next.js app with shadcn/ui components. Users can manage tasks, tweak settings (OpenAI model, proxy, concurrency), and view progress in real-time via a serial-polling mechanism. Architecturally, the application follows a monolith with clear separation of concerns: configuration, database, pipeline orchestration, and adapter modules for each ML component. The pipeline is modeled as a series of stages that can be run automatically or manually with step-by-step control. A notable design pattern is the use of adapters (e.g., `backend/app/adapters/`) that wrap third-party libraries and API calls, making the system modular and testable. The frontend uses a custom hook for race-free serial polling to update UI without conflicts. While primarily an application, it has strong research/data components: it bundles heavy ML models (Demucs, Whisper, VoxCPM2) and includes carefully crafted translation prompts for different language pairs (en→zh, ja→zh, zh→en). The pipeline is GPU-optimized with explicit memory management. The repository is hybrid—application with embedded ML research—but the dominant purpose is to serve as a production-ready tool for content creators, validated by a real Bilibili channel with 1M+ followers.