Locale Testing

ExTester supports launching the test VS Code instance in a non-English display language. This lets you verify that your extension’s UI strings, commands, and interactions behave correctly when VS Code itself is localized — for example, when running in Russian, Chinese, or French.

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How it works

VS Code’s display language is controlled by two files that must exist in its user-data directory before launch:

• User/locale.json — declares which locale to use (e.g. { "locale": "ru", "osLocale": "ru" })
• languagepacks.json — registry mapping locale codes to installed language pack extensions

When you pass the --locale / -L flag, ExTester writes User/locale.json automatically. languagepacks.json works differently: it is generated by the VS Code CLI when the language pack is installed into the test instance (via -i or an install-from-marketplace step). ExTester preserves that file across its settings-directory reset, but never generates it itself — which is why the language pack must be installed through ExTester rather than with a system code binary. ExTester also passes --locale=<locale> on the VS Code command line, which covers display-language behavior when languagepacks.json is missing.

All you need to do is declare the language pack as an extension dependency so it gets installed into the test instance.

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Step 1 — Declare the language pack as a dependency

Add the language pack extension ID to extensionDependencies in your extension’s package.json:

{
  "extensionDependencies": ["ms-ceintl.vscode-language-pack-es"]
}

Language pack extension IDs follow the pattern ms-ceintl.vscode-language-pack-<locale>. Some common ones:

Locale	Extension ID
Russian	ms-ceintl.vscode-language-pack-ru
Simplified Chinese	ms-ceintl.vscode-language-pack-zh-hans
Traditional Chinese	ms-ceintl.vscode-language-pack-zh-hant
French	ms-ceintl.vscode-language-pack-fr
German	ms-ceintl.vscode-language-pack-de
Japanese	ms-ceintl.vscode-language-pack-ja
Korean	ms-ceintl.vscode-language-pack-ko
Spanish	ms-ceintl.vscode-language-pack-es
Portuguese (Brazil)	ms-ceintl.vscode-language-pack-pt-br
Italian	ms-ceintl.vscode-language-pack-it

Find the full list on the VS Code Marketplace.

Important: extensionDependencies must only contain extensions that are available on the VS Code Marketplace. For offline or private setups see the Programmatic API section below.

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Step 2 — Run with -i and -L

Pass both flags together to setup-and-run (or run-tests):

extest setup-and-run './out/test/**/*.test.js' \
  -i \
  -L ru \
  -e ./test-extensions \
  -r .

Flag	Purpose
-i / --install_dependencies	Installs all extensionDependencies (including the language pack) into the test VS Code instance
-L <locale> / --locale <locale>	Launches VS Code with the given display language
-e <dir> / --extensions_dir <dir>	Directory where extensions are installed (optional)

Note: -L requires the language pack to already be installed into the test instance — otherwise VS Code falls back to English. Passing -i alongside -L is the convenient way to do that via extensionDependencies, but extest install-from-marketplace or the programmatic installFromMarketplace work equally well. Without -L, VS Code starts in English even if the pack is installed.

-e is optional. If you split setup and run into two commands, pass the same -e to both.

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npm script example

{
  "scripts": {
    "ui-test": "extest setup-and-run './out/test/**/*.test.js' -i -L ru -e ./test-extensions -r .",
    "ui-test:en": "extest setup-and-run './out/test/**/*.test.js' -i -e ./test-extensions -r ."
  }
}

It is good practice to keep your default ui-test script language-neutral and add a separate locale-specific variant only where needed, rather than hardcoding a locale for all test runs.

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Programmatic API

Use the locale field in RunOptions:

import { ExTester } from "vscode-extension-tester";

const exTester = new ExTester();

await exTester.setupAndRunTests(
  "./out/test/**/*.test.js",
  "latest",
  {
    installDependencies: true, // installs extensionDependencies, including the language pack
  },
  {
    resources: ["."],
    locale: "ru", // launches VS Code in Russian
  },
);

Or install the language pack explicitly without using extensionDependencies:

await exTester.installFromMarketplace("ms-ceintl.vscode-language-pack-es");

await exTester.runTests("./out/test/**/*.test.js", {
  resources: ["."],
  locale: "ru",
});

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CI example (GitHub Actions)

No extra install step is needed in CI. The -i flag handles installation through the test VS Code binary, which is the correct isolated instance:

- name: 🔍 Run localized tests
  run: extest setup-and-run './out/test/**/*.test.js' -i -L ru -e ./test-extensions -r .

Do not use code --install-extension in CI to install language packs — that installs into whatever system VS Code binary is on the PATH, not the isolated test instance that ExTester downloads and manages.

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Troubleshooting

VS Code still launches in English after setting -L ru

1. Make sure the language pack is installed into the test instance — via -i, extest install-from-marketplace, or the programmatic installFromMarketplace.
2. Make sure -e <dir> is consistent between setup and run — both the install command and the launch command must reference the same extensions directory.
3. Check that the language pack was actually downloaded: look for a ms-ceintl.vscode-language-pack-<locale>-* directory inside your -e folder after setup runs.

Language pack is installed but locale code doesn’t match

The locale code passed to -L must match the locale string the language pack declares. Russian is ru, Simplified Chinese is zh-cn (not zh-hans). Check the extension’s package.json → contributes.localizations[0].languageId for the exact string.