Express Locale Middleware

express-locale is an Express.js middleware designed to robustly determine the locale identifier for incoming HTTP requests. It supports a configurable priority list of lookup sources, including `accept-language` headers, query parameters, cookies, hostname mappings, and a default fallback. Since version 2.0.0, the package standardizes locale output using hyphen-separated format (e.g., `en-US`), aligning with JavaScript's `Intl` object, while still supporting underscore-formatted input for backward compatibility. The current stable version is 2.0.2. While there isn't a strict rapid release cadence, the project receives maintenance updates. Its key differentiators include a flexible configuration for lookup order, the ability to define custom lookup functions, and a clear distinction between language-only and full locale identifiers, with an optional mapping feature, allowing for precise locale resolution based on application needs.