var customers = {
    apple: {
        papa: {
            en: "cool"
        }
    },
    oranges: {
        papa: {
            en: "cool"
        }
    },
    test : "test",
    contacts : [{
    	nom: "test",
    	prenom : "testt"
    	},
    	{
    	nom : "test2",
    	prenom : "testt2"
    	}
    	]
};

function deepCopyEn(src) {


    if (src.hasOwnProperty("en")) {
      src.fr = src.en;
      src.es = src.en;
    }
    else {
        if (src.constructor === Array) {
            for (var i = 0; i < src.length; i++) {
                deepCopyEn(src[i]);
            }
        }
        else {
            for (var prop in src) {
                if(src.hasOwnProperty(prop)) {
                    deepCopyEn(src[prop]);
                }
            }
        }
    }
}

deepCopyEn(customers);

console.log(customers);