chore: update ping notification content (#3855)

lib/l10n/intl_en.arb

@@ -5266,5 +5266,17 @@
   "generatingSummary": "Analyzing chat and generating results",
   "instructionsLanguage": "Instructions language",
   "findCourse": "Find a course",
-  "activityCompletedDesc": "Your completed activity was added to analytics where you can review and practice the language you used."
+  "activityCompletedDesc": "Your completed activity was added to analytics where you can review and practice the language you used.",
+  "pingParticipantsNotification": "{user} is looking for users to join the activity session in {room}",
+  "@pingParticipantsNotification": {
+    "type": "String",
+    "placeholders": {
+      "user": {
+        "type": "String"
+      },
+      "room": {
+        "type": "String"
+      }
+    }
+  }
 }

lib/pangea/activity_sessions/activity_session_start/activity_session_start_page.dart

@@ -138,7 +138,12 @@ class ActivitySessionStartController extends State<ActivitySessionStartPage> {
       throw Exception("Activity is not part of a course");
     }
 
-    await room.courseParent!.sendTextEvent("");
+    await room.courseParent!.sendTextEvent(
+      L10n.of(context).pingParticipantsNotification(
+        room.client.userID!.localpart ?? room.client.userID!,
+        room.getLocalizedDisplayname(MatrixLocals(L10n.of(context))),
+      ),
+    );
   }
 
   @override