vue-i18n

Vue I18n is the de facto internationalization plugin for Vue.js 3, maintained under the Intlify project. It lets you define locale message catalogs, translate strings with the $t/t helpers, and localize dates, numbers, and currencies through the native Intl APIs.

It ships two API styles: a Composition API centered on useI18n() for <script setup> components, and a Legacy API that mirrors Vue I18n v8 for easier migration. Built on the Intlify message-compiler and core runtime, it supports named and list interpolation, linked messages, pluralization rules, and precompiled message functions for production performance.

What You Get

• A Vue 3 plugin installed via createI18n with reactive locale switching
• Composition API (useI18n) and Legacy API ($t, $tc, $d, $n) surfaces
• Message syntax with named/list interpolation, linked messages, and pluralization
• Locale-aware date, number, and currency formatting built on the Intl APIs
• Precompilable message functions and a CDN build for production use

Common Use Cases

• Localizing a Vue 3 single-page application into multiple languages
• Formatting dates, numbers, and currencies per the user’s active locale
• Handling pluralization and gendered/linked message variants across locales
• Migrating an existing Vue I18n v8 codebase to Vue 3 via the Legacy API

Under The Hood

Architecture - Vue I18n is a layered pnpm monorepo. At the base, @intlify/message-compiler parses the Intlify message syntax into an AST (ResourceNode -> MessageNode/PluralNode -> element nodes) and compiles it to message functions. @intlify/core-base provides the framework-agnostic runtime (translate, datetime, number, fallback-locale chaining). @intlify/vue-i18n-core builds the Vue integration on top: the Composer (packages/vue-i18n-core/src/composer.ts), the i18n instance and Vue plugin, and the <i18n-t>/<i18n-n>/<i18n-d> functional components. The public vue-i18n package (packages/vue-i18n/src/index.ts) is a thin entry re-exporting the core APIs plus feature-flag/dev initialization.

Tech Stack - Written in TypeScript (94% of the codebase) targeting Node >= 22, with Vue as a peer dependency. The build is driven by tsx scripts (scripts/build.ts) producing ESM, CommonJS, and global/CDN bundles; internal packages are wired through pnpm workspace protocol dependencies (workspace:*). Tooling includes Vitest for tests, oxlint plus ESLint for linting, Prettier/oxfmt for formatting, and a benchmark suite plus VitePress docs site.

Code Quality - The repo is thoroughly tested, with roughly 49 *.test.ts suites across packages plus a dedicated e2e/ directory and a formal message-syntax spec (spec/syntax.ebnf). Types are strict and exported extensively from the public entry, the message layer is benchmarked, and CI runs lint, type-aware checks, and tests. The consistent naming and clear package boundaries reflect a mature, well-maintained project.

API Design - The public API is deliberately ergonomic: createI18n for setup, the useI18n() composable for <script setup>, and the familiar t/d/n (plus “ legacy) helpers in templates. Offering both a Composition API and a v8-compatible Legacy API smooths migration, and the documentation site, examples directory, and CodeSandbox playground make onboarding straightforward for teams already comfortable with Vue.