This file is meant to be the base structure for this project, at least it’s offline usage. It will contains all your app strings and languages, even the ones your current plan can’t afford. You can either declare your strings and languages even before setting up your repository on dashboard or get it after creating your languages and strings by yourself there.

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Syntax

Unlikely other translation tools, such as .arb files, you declare all of your languages and strings in a single file called translation_fallback.json. It will be placed in a folder called translations, inside your /lib (for Flutter projects) or /src (on TS projects). As you can see in the example below, you can either declare you string values in string/string format or in an string/object one:
lib/translations/translation_callback.json
    {
        "en": {
            "myString01" :  "my string value",
            "myString02" : {
                "value" : "My object value"
            }
        },
        "pt": {
            "myString01" :  "Minha string",
            "myString02" : {
                "value" : "Meu objeto"
            }
        }
    }
You’ll mostly use object format to handle plural and none forms of an string

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Plural

By default, when you declare an string, using object format, which contains plural or none props, when this file get parsed to your sdk’s language you’ll get a param named count, which will define which one to show
lib/translations/translation_callback.json
    {
        "en": {
            "newMessagesToast" : {
                "value" : "You have a new message",
                "none" : "You have no new messages",
                "plural" : "You have new messages"
            }
        }
    }

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Arguments

You can add arguments to your strings by wrapping up your desired argument names with
lib/translations/translation_callback.json
    {
        "en": {
            "newMessagesToast" : {
                "value" : "{user}, you have a new message",
                "none" : "{user}, you have no new messages",
                "plural" : "{user}, you have new {count} messages"
            }
        }
    }