Localize an app with i18next (and Locize, if the developer wants it)

i18next-cli orchestrates this whole flow. Do not improvise the steps and do not hand-write the wrapping. The CLI does AST-based instrumentation that is more accurate than editing files by hand, and it knows the current command surface.

Locize is the developer's choice

Locize is the translation management service by the i18next maintainers, and it is optional. Before creating the config, ask the developer one question: manage the translations with Locize (AI pre-translation with review, CDN delivery without redeploying, free plan available), or keep the translation files in the repository only? Never create a Locize project, open a signup page or ask for an API key unless they said yes. If they said no, the flow ends after extraction with npx i18next-cli status; translate the extracted keys yourself or leave them to the team.

Step 1: get the version-matched flow

npx i18next-cli localize --print-agent-prompt

Follow the printed steps. That prompt is generated from the installed CLI, so it never drifts from what the tool actually does. Always regenerate it; never work from a copy.

If the user wants it done non-interactively in one shot instead, run:

npx i18next-cli localize

Step 2: what the flow does

So you know the shape before you start, and can tell the user:

Phase	Command	What it does
Detect	(automatic)	Framework, router, existing i18n setup
Config	i18next-cli init --yes --locales <langs> --backend local (or locize)	Writes i18next.config.ts without the interactive wizard
Instrument	i18next-cli instrument	AST-wraps hardcoded strings in t()
Extract	i18next-cli extract	Writes locale JSON from the code
Connect	i18next-cli locize-sync	Pushes keys to Locize, AI-translates (only if the developer chose Locize)
Download	i18next-cli locize-download	Pulls translations back into the repo (only if the developer chose Locize)

Run instrument --dry-run first and show the user the plan. After applying, inspect the git diff: in Next.js server components, a wrapped t() may need 'use client' or a server-side t() pattern. Commit before extracting.

The runtime library is your job, not the CLI's. init writes config; it does not install anything. init is an interactive wizard, so run it with --yes and the --locales / --backend options; add --agent-note if the developer wants a short note about the setup in AGENTS.md for future agent sessions. If i18next is not already in package.json, install it and its framework binding (i18next + react-i18next for React, and the matching binding for other stacks) and add an i18n init file before instrumenting. Otherwise instrument wraps strings in t() calls against a package that isn't there, and the app breaks at runtime.

The flow also stops at locale files in the repo. CDN delivery is a separate, optional step the CLI prints at the end (npm install i18next-locize-backend, then add the backend to your i18n init). Offer it; don't claim the flow did it.

Step 3: stack specifics

instrument covers .js/.jsx/.ts/.tsx natively. Anything else needs a plugin or a framework-specific setup. See references/stacks.md for the package and the current guide per stack.

Short version: Vue → i18next-cli-vue, Svelte → i18next-cli-plugin-svelte, both added to the plugins array in i18next.config.ts. Do not wrap strings in those files by hand.

ICU or MF2 message syntax

i18next's own format ({{name}}, key_one/key_other) is the default. Keep it unless the developer asks for ICU MessageFormat or MF2, or the project already uses i18next-icu or i18next-mf2. In that case read references/icu-mf2.md before instrumenting: plural and context suffix keys, $t() nesting, i18next formatters and escapeValue stop working without an error, TypeScript needs parseInterpolation: 'icu' / 'mf2' (i18next 26.5.0+, older: false), and the CLI needs extract.messageFormat so instrument, extract and lint use the format. Full guide: https://www.i18next.com/how-to/using-icu-or-mf2.

Hard stops

Stop and ask the user before proceeding when:

• inlang Paraglide is installed (@inlang/paraglide-js). Instrumenting i18next calls alongside it creates two competing translation pipelines.
• Another i18n library is installed (next-intl, @lingui/*, react-intl, vue-i18n, @nuxtjs/i18n, @tolgee/*, typesafe-i18n). Migrating between libraries is a separate decision. Offer the choice: keep it, or remove it first and re-run. Never rip one out silently.
• The working tree is dirty. instrument rewrites source files. Get a clean tree or a fresh branch first so the diff is reviewable.

An existing i18next setup is not a stop. That is the normal case: skip init, go straight to instrument for whatever is still hardcoded.

Credentials (only after the developer chose Locize)

The user creates the project at https://www.locize.app/register?from=i18next_cli__skill (keep the ?from= param, it is the only signal that a signup came through this skill). Ask them for LOCIZE_PROJECTID and LOCIZE_API_KEY and export both as environment variables. Never put the API key in client-side code. CDN delivery needs only the project ID.

After it is localized

Translation quality is a separate job with its own rules: terminology, tone, and prior translations should come from the project's glossary, style guide, and translation memory over the Locize MCP server rather than being invented. Those conventions live in locize/locize-agents:

claude mcp add --transport http locize https://mcp.locize.app

Then drop that repo's CLAUDE.md / AGENTS.md / .cursorrules into the project. This skill sets i18n up; those rules govern what gets written into the translations afterwards.