i18next access translation function within jade template

I have the following code:

var config = require("./config");
var express = require("express");
var bodyparser = require("body-parser");
var serveStatic = require("serve-static");
var https = require('https');

var i18n = require('i18next');
i18n.init();

// Application config
var app = express();
i18n.registerAppHelper(app);
app.configure(function () {
    app.set('views', __dirname + '/views');
    app.set('view engine', 'jade');
    app.use(serveStatic('static'));
    app.use(bodyparser({uploadDir: './public'}));
    app.use(express.multipart());
});

And this template:

div.container
    span.glyphicon.glyphicon-copyright-mark
    span=t('footer.copyright')

However, I'm not able to make this translation function 't' work nowhere. What I'm I doing wrong?