docs(fcm): add documentation for localized strings in notifications (#…

…9502)

* doc(fcm): add documentation for localized strings in notifications

* doc(fcm): add documentation for localized strings in notifications

* doc(fcm): add documentation for localized strings in notifications

docs/cloud-messaging/receive.md

@@ -273,3 +273,69 @@ class _Application extends State<Application> {
 ```
 
 How you handle interaction depends on your application setup. The above example shows a basic illustration using a StatefulWidget.
+
+## Localize Messages
+
+You can send localized strings in two different ways:
+
+* Store the preferred language of each of your users in your server and send customized notifications for each language
+* Embed localized strings in your app and make use of the operating system's native locale settings
+
+Here's how to use the second method:
+
+### Android
+
+1. Specify your default-language messages in `resources/values/strings.xml`:
+
+   ```xml
+   <string name="notification_title">Hello world</string>
+   <string name="notification_message">This is a message</string>
+   ```
+
+2. Specify the translated messages in the <code>values-<var>language</var></code> directory. For example, specify French messages in `resources/values-fr/strings.xml`:
+
+   ```xml
+   <string name="notification_title">Bonjour le monde</string>
+   <string name="notification_message">C'est un message</string>
+   ```
+
+3. In the server payload, instead of using `title`, `message`, and `body`  keys, use `title_loc_key` and `body_loc_key` for your localized message, and set them to the `name` attribute of the message you want to display.
+
+   The message payload would look like this:
+
+   ```json
+   {
+     "data": {
+       "title_loc_key": "notification_title",
+       "body_loc_key": "notification_message"
+     },
+   }
+   ```
+
+
+### iOS
+
+1. Specify your default-language messages in `Base.lproj/Localizable.strings`:
+
+   ```
+   "NOTIFICATION_TITLE" = "Hello World";
+   "NOTIFICATION_MESSAGE" = "This is a message";
+   ```
+
+2. Specify the translated messages in the <code><var>language</var>.lproj</code> directory. For example, specify French messages in `fr.lproj/Localizable.strings`:
+
+   ```
+   "NOTIFICATION_TITLE" = "Bonjour le monde";
+   "NOTIFICATION_MESSAGE" = "C'est un message";
+   ```
+
+   The message payload would look like this:
+
+   ```json
+   {
+     "data": {
+       "title_loc_key": "NOTIFICATION_TITLE",
+       "body_loc_key": "NOTIFICATION_MESSAGE"
+     },
+   }
+   ```