IOS: How to support old apps while adding a localisation to push notification

I would like to add new push notification to my app. It would have a localisation so lets imagine it looks like this.

{
    "aps" : {
        "alert" : {
            "loc-key" : "localization_key",
        }
    }
}

The version that will have this localization_key inside of .strings file is V2. How do i handle this notification on V1 that is already on the app store, as the push notification my user will see will have "localization_key" as body