Best Seo/language Settings For A Multistore

I am looking for this for quite a while, but what would be the best settings for the seo languages.

*Show language in url
*Use single url for all languages

Do you want the language to show in the url. If not selected it places a ?sl=EN behind every page/product.

Use single url for all languages
Is it even possible to use another url for another language? How does this work?