Document use of ENABLED_LANGUAGES (github#20991)

contributing/development.md

@@ -54,6 +54,12 @@ While running the local server, you can visit [localhost:4000/dev-toc](http://lo
 
 At the `/dev-toc` path, you'll see a list of available versions. Click a version, and a list of products will appear. Note that the TOC content is versioned. If you are viewing the `GitHub.com` version and you click the `Enterprise Admin` product, it will be empty, because there isn't any Admin content available on that version.
 
+### Enabling different languages
+
+By default the local server won't run with all supported languages enabled.  If you need to run the server with a particular language, you can temporarily edit the `start` script in `package.json` and update the `ENABLED_LANGUAGES` variable.  For example, to enable Japanese and Portuguese, you can set it to `ENABLED_LANGUAGES='en,ja,pt'` and then you need to restart the server for the change to take effect.
+
+The supported language codes are defined in [lib/lanuages.js](../lib/languages.js).
+
 ## Site structure
 
 This site was originally a Ruby on Rails web application. Some time later it was converted into a static site powered by [Jekyll](https://jekyllrb.com/). A few years after that it was migrated to [Nanoc](https://nanoc.ws/), another Ruby static site generator.