MM-11174: Fix /msg slash command. (#9059)

i18n/en.json

@@ -822,6 +822,10 @@
     "id": "api.command_remove.permission.app_error",
     "translation": "You do not have the appropriate permissions to remove the member."
   },
+  {
+    "id": "api.command_msg.permission.app_error",
+    "translation": "You don't have the appropriate permissions to direct message this user."
+  },
   {
     "id": "api.command_remove.user_not_in_channel",
     "translation": "{{.Username}} is not a member of this channel."