js-lingui

Lingui uses ICU MessageFormat, the same standard used by Android, iOS, and Java, for handling translations, plurals, and formatting. It compiles translations at build time, so your runtime bundle stays small.

MIT license, TypeScript. The key differentiator is the macro system: you write t`Hello {name}` directly in your code, and Lingui's compiler extracts messages automatically. No manual key management. Supports React, Vue, and plain JavaScript. The CLI extracts all translatable strings and generates catalogs.

Fully free. No paid tier, no hosted service. The entire toolchain (macros, CLI extractor, runtime, React bindings) is open source. Solo to medium teams: Lingui shines when you want automatic message extraction instead of maintaining key-value JSON files. Large teams: works well, but the smaller community means fewer third-party integrations compared to i18next.

The catch: the ecosystem is smaller. i18next has 10x the stars, more plugins, more tutorials, and wider adoption. If you're joining an existing team, they're almost certainly using i18next or FormatJS. Lingui is the better-designed library, but the network effects aren't there yet.