title: How to insert a placeholder in a json key:value ? Flutter
body: <p>I'm porting my Swift app to Flutter and for localising it I'm following this <a href="https://github.com/billylev/flutter_localizations" rel="nofollow noreferrer">https://github.com/billylev/flutter_localizations</a> but I can't see how to insert placeholder to insert a value in the translated values. 
  Basically the guide uses </p>
  
  <pre><code>String text(String key) {
      return _localisedValues[key] ?? "$key not found";
    } 
  </code></pre>
  
  <p>to get the corresponding key:value pair from a .json file as </p>
  
  <pre><code>{
    "Shop": "Negozio",
  }
  </code></pre>
  
  <p>I just pass it in the <code>Text</code>widget as :</p>
  
  <p><code>Text(AppLocalizations.instance.text('Shop'))</code>.</p>
  
  <p>How to modify <code>text</code> to insert one or more placeholders and how would be the .json be constructed? 
  Say for the value <code>"User": "User"</code> I'd like to insert a value after the transaction I can simply use a string sum and add the value as `Text(
                              AppLocalizations.instance.text('User') + ' ${widget.user.name}', but if I need to insert a value in the middle of the translated sentence, eg a message, I don't see how to accomplish it.
  I need it to make localised versions of incoming push notification, and they have args.</p>
  
  <p>In Swift I have it like this:</p>
  
  <pre><code>"ORDER_RECEIVED_PUSH_TITLE" = "Order number: %@";
  "ORDER_RECEIVED_PUSH_SUBTITLE" = "Shop: %@";
  "ORDER_RECEIVED_PUSH_BODY" = "Thank you %@! We received your order and we'll let you know when we start preparing it and when it's ready. Bye";
  </code></pre>
  
  <p>Any suggestions on how to accomplish that in Flutter?</p>
  
  <p>Many thanks</p>