Document process of adding a new translation

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 - [How to add custom build to Chrome](./docs/add-to-chrome.md)
 - [How to add custom build to Firefox](./docs/add-to-firefox.md)
 - [How to develop a live-reloading UI](./docs/ui-dev-mode.md)
+- [How to add a new translation to MetaMask](./docs/translating-guide.md)
 - [Publishing Guide](./docs/publishing.md)
 - [How to develop an in-browser mocked UI](./docs/ui-mock-mode.md)
 - [How to live reload on local dependency changes](./docs/developing-on-deps.md)
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+# MetaMask Translation Guide
+
+The MetaMask browser extension supports new translations added in the form of new locales files added in `app/_locales`.
+
+- [The MDN Guide to Internationalizing Extensions](https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Internationalization)
+
+## Adding a new Language
+
+Each supported language is represented by a folder in `app/_locales` whose name is that language's subtag ([look up a language subtag using this tool](https://r12a.github.io/app-subtags/)).
+
+Inside that folder there should be a `messages.json` file that follows the specified format. An easy way to start your translation is to first duplicate `app/_locales/en/messages.json` (the english translation), and then update the `message` key for each in-app message.
+
+That's it! When MetaMask is loaded on a computer with that language set as the system language, they will see your translation instead of the default one.
+
+## Testing
+
+To verify that your translation works, you will need to [build a local copy](https://github.com/MetaMask/metamask-extension#building-locally) of MetaMask.
+