Injected

function handleMessage(event) {
    if (event.name === "getHighlightedText") {
        var textSelectedForTranslation = document.getSelection().toString();
        alert("Me!");
        safari.self.tab.dispatchMessage("translateThis",textSelectedForTranslation);
    }
    if (event.name === "displayTranslation") {
        alert(event.message);
    }
}

safari.self.addEventListener("message", handleMessage, false);