Updated Crowdin README

Author: Brian Vaughn

crowdin/README.md

@@ -74,4 +74,23 @@ This configuration is done via `gatsby-config.js`:
 
 Because of the default initial symlink (`crowdin/__translated__/en-US/docs` -> `content/docs`) Gatsby will still serve English content when run locally, even if the Crowdin script has not been run. This should enable fast iteration and creation of new content.
 
-Translations can be updated by running `yarn crowdin:download` (or automatically as part of CI deployment).
+Translations can be updated by running `yarn crowdin:download` (or automatically as part of CI deployment).
+
+### Language selector
+
+The Yarn task `crowdin:update-languages` determines which translated languages have been downloaded. (This task is automatically run before `yarn dev` or `yarn build` in order to just-in-time update the list.) The task writes a list of locales to a local JSON file, `languages.json`:
+
+```sh
+.
+└── crowdin
+   ├── __exported__
+   │   └── # ...
+   ├── __translated__
+   │   └── # ...
+   ├── __untranslated__
+   │   └── # ...
+   ├── translated-languages.json # This is the list of local translations
+   └── # ...
+```
+
+This `languages.json` list is imported into a translations page (`pages/translations.js`) and used to create a list of links to translated docs.