lib: add navigator.language and navigator.languages

doc/api/globals.md

@@ -637,6 +637,47 @@ logical processors available to the current Node.js instance.
 console.log(`This process is running on ${navigator.hardwareConcurrency} logical processors`);
 ```
 
+### `navigator.language`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* {string}
+
+The `navigator.language` read-only property returns a string representing the
+preferred language of the Node.js instance. The language will be determined by
+the ICU library used by Node.js at runtime based on the
+default language of the operating system.
+
+The value is representing the language version as defined in [RFC 5646][].
+
+The fallback value on builds without ICU is `'en-US'`.
+
+```js
+console.log(`The preferred language of the Node.js instance has the tag '${navigator.language}'`);
+```
+
+### `navigator.languages`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* {Array<string>}
+
+The `navigator.languages` read-only property returns an array of strings
+representing the preferred languages of the Node.js instance.
+By default `navigator.languages` contains only the value of
+`navigator.language`, which will be determined by the ICU library used by
+Node.js at runtime based on the default language of the operating system.
+
+The fallback value on builds without ICU is `['en-US']`.
+
+```js
+console.log(`The preferred languages are '${navigator.languages}'`);
+```
+
 ### `navigator.platform`
 
 <!-- YAML
@@ -1100,6 +1141,7 @@ A browser-compatible implementation of [`WritableStreamDefaultWriter`][].
 [CommonJS module]: modules.md
 [ECMAScript module]: esm.md
 [Navigator API]: https://html.spec.whatwg.org/multipage/system-state.html#the-navigator-object
+[RFC 5646]: https://www.rfc-editor.org/rfc/rfc5646.txt
 [Web Crypto API]: webcrypto.md
 [`--experimental-websocket`]: cli.md#--experimental-websocket
 [`--no-experimental-global-customevent`]: cli.md#--no-experimental-global-customevent

lib/internal/navigator.js

@@ -5,9 +5,15 @@ const {
   StringPrototypeIndexOf,
   StringPrototypeSlice,
   StringPrototypeToUpperCase,
+  ObjectFreeze,
+  globalThis,
   Symbol,
 } = primordials;
 
+const {
+  Intl,
+} = globalThis;
+
 const {
   ERR_ILLEGAL_CONSTRUCTOR,
 } = require('internal/errors').codes;
@@ -74,6 +80,8 @@ class Navigator {
   #availableParallelism;
   #userAgent = `Node.js/${StringPrototypeSlice(nodeVersion, 1, StringPrototypeIndexOf(nodeVersion, '.'))}`;
   #platform = getNavigatorPlatform(process);
+  #language = Intl?.Collator().resolvedOptions().locale || 'en-US';
+  #languages = ObjectFreeze([this.#language]);
 
   constructor() {
     if (arguments[0] === kInitialize) {
@@ -90,6 +98,20 @@ class Navigator {
     return this.#availableParallelism;
   }
 
+  /**
+   * @return {string}
+   */
+  get language() {
+    return this.#language;
+  }
+
+  /**
+   * @return {Array<string>}
+   */
+  get languages() {
+    return this.#languages;
+  }
+
   /**
    * @return {string}
    */
@@ -107,6 +129,8 @@ class Navigator {
 
 ObjectDefineProperties(Navigator.prototype, {
   hardwareConcurrency: kEnumerableProperty,
+  language: kEnumerableProperty,
+  languages: kEnumerableProperty,
   userAgent: kEnumerableProperty,
   platform: kEnumerableProperty,
 });

test/parallel/test-navigator.js

@@ -1,10 +1,11 @@
-'use strict';
-
 // Flags: --expose-internals
 
-require('../common');
+'use strict';
+
+const common = require('../common');
 const assert = require('assert');
 const { getNavigatorPlatform } = require('internal/navigator');
+const { execFile } = require('child_process');
 
 const is = {
   number: (value, key) => {
@@ -74,3 +75,48 @@ assert.strictEqual(getNavigatorPlatform({ arch: 'ia32', platform: 'sunos' }), 'S
 assert.strictEqual(getNavigatorPlatform({ arch: 'x64', platform: 'sunos' }), 'SunOS x64');
 assert.strictEqual(getNavigatorPlatform({ arch: 'ppc', platform: 'aix' }), 'AIX');
 assert.strictEqual(getNavigatorPlatform({ arch: 'x64', platform: 'reactos' }), 'Reactos x64');
+
+assert.strictEqual(typeof navigator.language, 'string');
+assert.strictEqual(navigator.language.length !== 0, true);
+
+assert.ok(Array.isArray(navigator.languages));
+assert.strictEqual(navigator.languages.length, 1);
+assert.strictEqual(typeof navigator.languages[0], 'string');
+assert.strictEqual(navigator.languages[0].length !== 0, true);
+
+assert.throws(() => {
+  navigator.languages[0] = 'foo';
+}, new TypeError("Cannot assign to read only property '0' of object '[object Array]'"));
+assert.notStrictEqual(navigator.languages[0], 'foo');
+assert.strictEqual(typeof navigator.languages[0] === 'string', true);
+assert.strictEqual(navigator.languages[0].length !== 0, true);
+
+if (common.hasIntl && common.isWindows === false) {
+  const testLocale = navigator.language === 'de-DE' ?
+    'en-US' :
+    'de-DE';
+  {
+    const env = { ...process.env, LC_ALL: testLocale };
+    execFile(
+      process.execPath,
+      ['--print', `"process.exit(navigator.language === '${testLocale}' ? 0 : 1)"`],
+      { env },
+      common.mustSucceed()
+    );
+  }
+
+  {
+    const env = { ...process.env, LC_ALL: testLocale };
+    execFile(
+      process.execPath,
+      ['--print', `"process.exit(navigator.languages[0] === '${testLocale}' ? 0 : 1)"`],
+      { env },
+      common.mustSucceed()
+    );
+  }
+}
+
+Object.defineProperty(navigator, 'language', { value: 'for-testing' });
+assert.strictEqual(navigator.language, 'for-testing');
+assert.strictEqual(navigator.languages.length, 1);
+assert.strictEqual(navigator.languages[0] !== 'for-testing', true);