diff --git a/package.json b/package.json
index 2fde3ab..318fa93 100644
--- a/package.json
+++ b/package.json
@@ -5,7 +5,7 @@
   "description": "Verstak browser capture extension for Chromium and Firefox",
   "scripts": {
     "build": "node scripts/build-extension.js",
-    "test": "node scripts/test-protocol.js && node scripts/test-popup-settings.js",
+    "test": "node scripts/test-protocol.js && node scripts/test-i18n.js && node scripts/test-popup-settings.js",
     "sign:firefox": "./scripts/sign-firefox-xpi.sh",
     "release:firefox": "./scripts/release-firefox-xpi.sh"
   },
diff --git a/scripts/test-i18n.js b/scripts/test-i18n.js
new file mode 100644
index 0000000..6a7ade6
--- /dev/null
+++ b/scripts/test-i18n.js
@@ -0,0 +1,45 @@
+#!/usr/bin/env node
+const assert = require('assert');
+const fs = require('fs');
+const path = require('path');
+
+const root = path.resolve(__dirname, '..');
+const i18n = require(path.join(root, 'shared', 'i18n.js'));
+const en = JSON.parse(fs.readFileSync(path.join(root, 'shared', 'locales', 'en.json'), 'utf8'));
+const ru = JSON.parse(fs.readFileSync(path.join(root, 'shared', 'locales', 'ru.json'), 'utf8'));
+
+assert.strictEqual(i18n.normalizePreference(undefined), 'system');
+assert.strictEqual(i18n.normalizePreference('de'), 'system');
+assert.strictEqual(i18n.normalizePreference('system'), 'system');
+assert.strictEqual(i18n.normalizePreference('en'), 'en');
+assert.strictEqual(i18n.normalizePreference('ru'), 'ru');
+
+assert.strictEqual(i18n.resolveLocale('system', 'ru'), 'ru');
+assert.strictEqual(i18n.resolveLocale('system', 'ru-RU'), 'ru');
+assert.strictEqual(i18n.resolveLocale('system', 'RU-ru'), 'ru');
+assert.strictEqual(i18n.resolveLocale('system', 'uk-UA'), 'en');
+assert.strictEqual(i18n.resolveLocale('system', 'en-US'), 'en');
+assert.strictEqual(i18n.resolveLocale('en', 'ru-RU'), 'en');
+assert.strictEqual(i18n.resolveLocale('ru', 'en-US'), 'ru');
+
+assert.deepStrictEqual(Object.keys(ru).sort(), Object.keys(en).sort());
+assert.ok(Object.values(en).every((value) => typeof value === 'string'));
+assert.ok(Object.values(ru).every((value) => typeof value === 'string'));
+
+const tEn = i18n.createTranslator({ en, ru }, 'en');
+const tRu = i18n.createTranslator({ en, ru }, 'ru');
+assert.strictEqual(tRu('status.queued'), 'В очереди до запуска Верстака');
+assert.strictEqual(tEn('error.value', { error: 'offline' }), 'Error: offline');
+assert.strictEqual(tRu('missing', null, 'Fallback'), 'Fallback');
+assert.strictEqual(tRu('missing.key'), 'missing.key');
+
+i18n.loadCatalogs((locale) => Promise.resolve(locale === 'ru' ? ru : en))
+  .then((catalogs) => {
+    assert.strictEqual(catalogs.en, en);
+    assert.strictEqual(catalogs.ru, ru);
+    console.log('browser extension localization runtime tests passed');
+  })
+  .catch((error) => {
+    console.error(error);
+    process.exit(1);
+  });
diff --git a/shared/i18n.js b/shared/i18n.js
new file mode 100644
index 0000000..c8192bc
--- /dev/null
+++ b/shared/i18n.js
@@ -0,0 +1,54 @@
+(function (root, factory) {
+  'use strict';
+
+  var api = factory();
+  root.VerstakBrowserI18n = api;
+  if (typeof module !== 'undefined' && module.exports) module.exports = api;
+})(typeof globalThis !== 'undefined' ? globalThis : this, function () {
+  'use strict';
+
+  var PREFERENCES = { system: true, en: true, ru: true };
+
+  function normalizePreference(value) {
+    value = String(value || '').trim().toLowerCase();
+    return PREFERENCES[value] ? value : 'system';
+  }
+
+  function resolveLocale(preference, systemLocale) {
+    preference = normalizePreference(preference);
+    if (preference === 'en' || preference === 'ru') return preference;
+    systemLocale = String(systemLocale || '').trim().toLowerCase();
+    return systemLocale === 'ru' || systemLocale.indexOf('ru-') === 0 ? 'ru' : 'en';
+  }
+
+  function interpolate(message, params) {
+    if (!params) return message;
+    return message.replace(/\{([A-Za-z0-9_.-]+)\}/g, function (placeholder, name) {
+      return Object.prototype.hasOwnProperty.call(params, name) ? String(params[name]) : placeholder;
+    });
+  }
+
+  function loadCatalogs(loadJSON) {
+    return Promise.all([loadJSON('en'), loadJSON('ru')]).then(function (catalogs) {
+      return { en: catalogs[0] || {}, ru: catalogs[1] || {} };
+    });
+  }
+
+  function createTranslator(catalogs, locale) {
+    catalogs = catalogs || {};
+    locale = locale === 'ru' ? 'ru' : 'en';
+    return function translate(key, params, fallback) {
+      var message = catalogs[locale] && catalogs[locale][key];
+      if (message == null && catalogs.en) message = catalogs.en[key];
+      if (message == null) message = fallback == null ? key : fallback;
+      return interpolate(String(message), params);
+    };
+  }
+
+  return {
+    normalizePreference: normalizePreference,
+    resolveLocale: resolveLocale,
+    loadCatalogs: loadCatalogs,
+    createTranslator: createTranslator
+  };
+});
diff --git a/shared/locales/en.json b/shared/locales/en.json
new file mode 100644
index 0000000..5785e83
--- /dev/null
+++ b/shared/locales/en.json
@@ -0,0 +1,33 @@
+{
+  "popup.subtitle": "Browser Inbox",
+  "label.receiver": "Receiver",
+  "label.pending": "Pending",
+  "label.url": "URL",
+  "label.file": "File",
+  "label.receiverUrl": "Receiver URL",
+  "label.pairingToken": "Pairing token",
+  "label.language": "Language",
+  "action.sendPage": "Send Page",
+  "action.sendFile": "Send File",
+  "action.retryPending": "Retry Pending",
+  "action.save": "Save",
+  "hint.contextMenu": "Selection and link captures are available from the page context menu.",
+  "receiver.online": "Online",
+  "receiver.offline": "Offline",
+  "receiver.unknown": "Unknown",
+  "language.system": "System",
+  "language.en": "English",
+  "language.ru": "Russian",
+  "status.sending": "Sending...",
+  "status.queued": "Queued until Verstak is available",
+  "status.done": "Done",
+  "status.readingFile": "Reading file...",
+  "status.saved": "Saved",
+  "error.chooseFile": "Choose a file first",
+  "error.fileTooLarge": "File is too large for browser capture",
+  "error.invalidReceiverUrl": "Receiver URL must start with http:// or https://",
+  "error.value": "Error: {error}",
+  "context.sendPage": "Send page to Verstak",
+  "context.sendSelection": "Send selection to Verstak",
+  "context.sendLink": "Send link to Verstak"
+}
diff --git a/shared/locales/ru.json b/shared/locales/ru.json
new file mode 100644
index 0000000..059ef08
--- /dev/null
+++ b/shared/locales/ru.json
@@ -0,0 +1,33 @@
+{
+  "popup.subtitle": "Входящие из браузера",
+  "label.receiver": "Приёмник",
+  "label.pending": "В очереди",
+  "label.url": "URL",
+  "label.file": "Файл",
+  "label.receiverUrl": "URL приёмника",
+  "label.pairingToken": "Токен сопряжения",
+  "label.language": "Язык",
+  "action.sendPage": "Отправить страницу",
+  "action.sendFile": "Отправить файл",
+  "action.retryPending": "Повторить отправку",
+  "action.save": "Сохранить",
+  "hint.contextMenu": "Выделение и ссылки можно отправить через контекстное меню страницы.",
+  "receiver.online": "Доступен",
+  "receiver.offline": "Недоступен",
+  "receiver.unknown": "Неизвестно",
+  "language.system": "Системный",
+  "language.en": "Английский",
+  "language.ru": "Русский",
+  "status.sending": "Отправка...",
+  "status.queued": "В очереди до запуска Верстака",
+  "status.done": "Готово",
+  "status.readingFile": "Чтение файла...",
+  "status.saved": "Сохранено",
+  "error.chooseFile": "Сначала выберите файл",
+  "error.fileTooLarge": "Файл слишком велик для отправки из браузера",
+  "error.invalidReceiverUrl": "URL приёмника должен начинаться с http:// или https://",
+  "error.value": "Ошибка: {error}",
+  "context.sendPage": "Отправить страницу в Верстак",
+  "context.sendSelection": "Отправить выделение в Верстак",
+  "context.sendLink": "Отправить ссылку в Верстак"
+}