translate-default attribute for a placeholder

Question

I have included a directive for showing placeholder in my html and datasource attribute holds the key value for my translation

<input type="text" id="txtusername" placeholder-link datasource='{"pHolderKey":"81","pHolderMessage":"User Name"}' />

And the key value for the translations are in json files as below

en-US.json

{"81":"User Name"}

de-DE.json

{"81":"Benutzername"}

And below is my directive

app.directive("placeholderLink", function ($compile, $translate, $filter) {
        return {
            
            priority: 1001, // compiles first
            terminal: true, // prevent lower priority directives to compile after it
            compile: function (el) {
                var params = $.parseJSON(el.context.attributes['dataSource'].value);
                var key = params.pHolderKey;
                var value = $filter('translate')(key);
                var att = "";
                if (value == key)
                    att = params.pHolderMessage;                    
                else
                    att = "{{'" + key + "'|translate}}";
                el.removeAttr('placeholder-Link');
                el.attr('placeholder', att);
                var fn = $compile(el);
                return function (scope) {
                    fn(scope);
                };
            }
        };
    });

I have a dropdown to change the language between en-US and de-DE (Defualt languafe set to en-US). It works fine if we have the json data (en-US & de-DE) for the key 81.

If the key is available in json, the html will be as below

<input type="text" id="txtusername" placeholder="{{'81'|translate}}" />

and the translation works fine while changing the languages.

If the key is not available in json file for en-US, but available in de-DE. Then the html is formed as below

<input type="text" id="txtusername" placeholder="User Name" />

In this case, translation wont work for rest of the languages even if the key is available in de-DE json.

I need an attribute like translate-default for the placeholder.

Is it possible.

Thanks in Advance