Context.contentLanguage property (apps for Office)

function sayHelloWithContentLanguage() {
    var myContentLanguage = Office.context.contentLanguage;
    switch (myContentLanguage) {
        case 'en-US':
            write('Hello!');
            break;
        case 'en-NZ':
            write('G\'day mate!');
            break;
    }
}
// Function that writes to a div with id='message' on the page.
function write(message){
    document.getElementById('message').innerText += message; 
}