You need to send the request for translation from javascript, and get the results back as JSON (this should help: https://developers.google.com/translate/v2/using_rest). After that you need to assign the result to the autocomplete function, the code should be something like:
$('#textbox1').autocomplete({
source: function (request, response) {
$.getJSON("https://www.googleapis.com/language/translate/v2/languages?key" + request.term, function (data) {
response($.map(data, function (value, key) {
return { label: value, value: key
};
}));
});
},
minLength: 3,
delay: 25
});