Internationalization (i18n) / Localization for Node.js web application

I have a web application build using Node.js. I wish to do internationalization and support english and Spanish language.

Whenever the user selects the english link the static content of my 'index.ejs' page should be in english and when he selects Spanish it should change accordingly.

How can I do that? What would be the simplest way to do that? Also, How can I improve Search Engine Optimization (SEO)?