Multi-Language Site: Cookie or URL Element
Best practice for SEO is to have it in your url string /en/ /fr/ /es/. Else your website will only be indexed in one language, or some pages in one language, and other in another language.
It is better to have every different content (page+language) on a different page to avoid confusion by search engines.
It is also good practice to set the language in your meta:
<meta http-equiv="Content-Language" content="EN"/>