How to implement localization and multi-language support in a browser extension user interface?

Extension UI localization is a crucial step to enter the international market and improve usability for users from different countries. Modern browsers provide a standardized internationalization system that automatically detects the user's system language and adjusts interface text elements accordingly.

The main tool for implementing translation is a folder named `_locales`, which must be located in the root directory of the extension project. Inside this folder, subfolders are created for each supported language using standard language codes, such as `en` for English or `ru` for Russian, and translation files in JSON format are placed within them.

Each such file contains a set of keys and their corresponding translated strings with the ability to substitute dynamic variables. In the extension manifest and all interface HTML files, instead of plain text, special localization function calls are used, which are replaced by the browser with the correct translation during element rendering.

Properly configured internationalization not only makes the product accessible to a global audience but also simplifies further code maintenance. If you need to change text or fix a typo, the developer will not have to rewrite the interface logic; it will be sufficient to simply update the corresponding value in the translation file for the specific language.