var x = {
    'USD': 'DOLLAR',
    'JPY': 'JEN',
    'EUR': 'EURO'
};


var  y = [
    'USD 50',
    'JPY 20, USD 20',
    'EUR 20, JPY 10'
];

var z = y[Math.floor(Math.random() * y.length)]; //example 'JPY 20, USD 20'

for(var key in x) {
    var c = new RegExp({key}, "g");
    z = z.replace(key, x[key]);
    console.log(z);      //It should looks like //'JEN 20, DOLLAR 20'
}