cleanup: finish removing gulp (#163)

While there, sync README.md instructions with recent changes.

Author: bassosimone

README.md

@@ -13,16 +13,17 @@
 >
 > We expect all contributions to reflect a genuine understanding of the code being changed. PRs that appear to be generated by AI tools without meaningful human review will be closed.
 
-`mlab-speedtest` is an Angular.js application providing the website https://speed.measurementlab.net. The app code currently uses `gulp` to integrate language localizations prior to deployment.
+The `mlab-speedtest` repository implements the https://speed.measurementlab.net/ website.
 
 ## Setting up Local Development Environment
 
 ### Install Build Dependencies
 
 * Install website build dependencies for your operating system and environment
-  * Node >= v10.15.1 - Using NVM: `nvm install v10.15.1`
-  * Gulp - Installed with `yarn`
-  * Install libraries - `gulp copy_libs`
+  * Node >= v20.0 - Using NVM: `nvm install v20.0`
+  * Install libraries - `npm install`
+  * Build dist - `npm run build`
+  * Serving locally - `python3 -m http.server -d dist`
   * Firebase tools - `npm install -g firebase-tools`
 
 ## Adding new languages
@@ -31,25 +32,24 @@ Translations for this site are managed in the [Open Technology Fund's Localizati
 
 ### How to add a new localization
 
-* Download a completed language translation file
-  * Visit the Transifex site and locate a completed language
-  * Click **Interface Language Strings** and then **Download for use** to download the completed language `.po` file
-  * Save it in the folder: `/translations/source/` using the file name pattern: **<two letter langugage code>.po**
-* Create new language template & update language string references
-  * Create a copy of an existing translation folder and index.html file, and make some edits to support the new language
-    * Copy the folder and index file: `cp -r app/nl app/es`
-    * Edit the index file to change the language it references. In `app/es/index.html`: 
-      * change: `const INTERFACE_LANGUAGE = 'nl';` to `const INTERFACE_LANGUAGE = 'es';`
-      * change: `<script src="/assets/translations/it.js"></script>`
-    * to: `<script src="/assets/translations/es.js"></script>`
-  * Regenerate the supported language strings: 
-    * `gulp inject`
+1. Download the completed `.po` file from Transifex and save it in `translations/languages/`
+   * Use the existing naming convention (e.g., `es.po`, `de_DE.po`)
+2. If the filename uses a locale code (e.g., `de_DE`), add a mapping in `scripts/po-to-json.js` so it maps to the short code (e.g., `de_DE` -> `de`)
+3. Add the short language code to the `supported` array in `src/js/i18n.js`
+   * If the language is RTL, also add it to the `rtlLanguages` array
+4. Rebuild: `npm run build`
+
+The build converts `.po` files to JSON in `dist/translations/`. At runtime, `i18n.js` selects the language automatically via the `?lang=` query parameter, `localStorage`, or browser preference.
 
 ## Previewing site locally
 
-To preview the site locally, we recommend using the Python Simple HTTP Server. 
+To preview the site locally, we recommend using Python:
 
-* Navigate to the `/app` directory and run: `python3 -m http.server 8000`
+```
+npm install
+npm run build
+python3 -m http.server -d dist
+```
 
 If you are a user on the M-Lab Firebase project, you can also preview the site locally using the firebase-cli: `firebase serve --only hosting:mlab-speedtest`