fix(i18n): fix CI build breakage from prior commit

The previous commit added "type": "module" to packages/i18n/package.json
to silence a Vite native configLoader warning about ESM syntax loaded
as CommonJS. That broke this package's build pipeline: its tsx-invoked
scripts (build:split, build:compile-strings, build:dictionaries) use
CommonJS __dirname without an ESM fallback, and tsx treats a file as
ESM once the nearest package.json sets "type": "module". Every script
failed with `ReferenceError: __dirname is not defined`, no *.messages.ts
files got generated, and packages/nimbus's build then failed with
~36 "Cannot find module '*.messages'" errors.

Fix by reverting the package-wide "type": "module" and instead
renaming just the offending file to locales.mts (same approach already
used for the root-level vitest configs), which makes Vite's native
config loader treat it as ESM without touching how tsx resolves every
other file in the package. Updated the file's extensionless importers
(compile-component-messages.ts, extract-messages.ts,
generate-dictionaries.ts, split-by-component.ts) to the explicit
.mts extension, since tsx's CJS-mode resolver doesn't probe for .mts
on extensionless specifiers. Also updated the one cross-package import
in packages/nimbus/vite.config.ts and doc references
(packages/i18n/CLAUDE.md, packages/i18n/README.md).

Verified: pnpm --filter @commercetools/nimbus-i18n build succeeds,
pnpm --filter @commercetools/nimbus build succeeds (the exact command
that failed in CI), and the unit/unit-isolated/ts-plugin/nimbus-mcp/
scripts vitest projects all pass (2019 tests).

Author: ByronDWall

packages/i18n/CLAUDE.md

@@ -68,7 +68,7 @@ packages/i18n/
 │   ├── split-by-component.ts
 │   ├── compile-component-messages.ts
 │   ├── generate-dictionaries.ts
-│   └── locales.ts
+│   └── locales.mts
 ├── .temp/             # Temporary build artifacts (gitignored)
 │   └── by-component/  # Messages grouped by component
 ├── package.json
@@ -286,7 +286,7 @@ from the generated `*.messages.ts` files.
 
 ## Supported Locales
 
-Available locales (defined in `scripts/locales.ts`):
+Available locales (defined in `scripts/locales.mts`):
 
 - `en` - English
 - `de` - German
@@ -300,7 +300,7 @@ English for unsupported locales.
 
 To add a new locale:
 
-1. Add locale code to `scripts/locales.ts`
+1. Add locale code to `scripts/locales.mts`
 2. Create `data/[locale].json` file with Transifex translations
 3. Update Vite config's `optimize-locales-plugin` to include new locale
 4. Run build to compile: `pnpm --filter @commercetools/nimbus-i18n build`

packages/i18n/README.md

@@ -33,7 +33,7 @@ This is an **internal build tool package** that:
 ### Locale Configuration
 
 **Single Source of Truth:** All supported locales are defined in
-`packages/i18n/scripts/locales.ts` and shared between the build scripts and the
+`packages/i18n/scripts/locales.mts` and shared between the build scripts and the
 Vite config (`optimize-locales-plugin`).
 
 ### Build Pipeline

packages/i18n/package.json

@@ -12,7 +12,6 @@
   "keywords": ["typescript", "design-system", "react", "nimbus"],
   "license": "MIT",
   "private": true,
-  "type": "module",
   "sideEffects": false,
   "files": ["data"],
   "scripts": {

packages/i18n/scripts/compile-component-messages.ts

@@ -32,7 +32,7 @@ import { compileStrings } from "@internationalized/string-compiler";
 import fs from "fs/promises";
 import path from "path";
 import { format } from "prettier";
-import { LOCALE_CODES } from "./locales";
+import { LOCALE_CODES } from "./locales.mts";
 
 const LOCALES = LOCALE_CODES;
 

packages/i18n/scripts/extract-messages.ts

@@ -8,7 +8,7 @@
 import { glob } from "glob";
 import * as fs from "fs";
 import * as path from "path";
-import { LOCALE_CODES } from "./locales";
+import { LOCALE_CODES } from "./locales.mts";
 
 // Type definitions for message descriptors in .i18n.ts files
 type MessageDescriptor = {

packages/i18n/scripts/generate-dictionaries.ts

@@ -37,7 +37,7 @@
 import fs from "fs/promises";
 import path from "path";
 import { format, resolveConfig } from "prettier";
-import { SUPPORTED_LOCALES } from "./locales";
+import { SUPPORTED_LOCALES } from "./locales.mts";
 
 const LOCALES = SUPPORTED_LOCALES;
 

packages/i18n/scripts/locales.ts packages/i18n/scripts/locales.mtspackages/i18n/scripts/locales.ts renamed to packages/i18n/scripts/locales.mts

@@ -42,7 +42,7 @@
 
 import fs from "fs/promises";
 import path from "path";
-import { LOCALE_CODES } from "./locales";
+import { LOCALE_CODES } from "./locales.mts";
 
 const LOCALES = LOCALE_CODES;
 

packages/i18n/scripts/split-by-component.ts

@@ -6,7 +6,7 @@ import type { LibraryFormats, PluginOption, Rollup } from "vite";
 import react from "@vitejs/plugin-react";
 import dts from "vite-plugin-dts";
 import { analyzer } from "vite-bundle-analyzer";
-import { LOCALE_BCP47_CODES } from "../i18n/scripts/locales.ts";
+import { LOCALE_BCP47_CODES } from "../i18n/scripts/locales.mts";
 
 /**
  * Builds the entry map for the library build.
@@ -156,7 +156,7 @@ export default defineConfig(async () => {
     plugins: [
       react(),
       // Only package locale strings for locales we internationalize in our products
-      // Locales are defined in packages/i18n/scripts/locales.ts (single source of truth)
+      // Locales are defined in packages/i18n/scripts/locales.mts (single source of truth)
       // https://github.com/commercetools/merchant-center-application-kit/blob/main/packages/i18n/README.md#supported-locales
       // https://react-spectrum.adobe.com/react-aria/internationalization.html#vite
       optimizeLocales.vite({