SEO - localized url - best pattern [duplicate]

Question

I want to support languages (fr,en,de). My home page:

https://example.com/ (content localized based on webBrowser settings)

Language selection links available to set language in a cookie:

http://example.com/en 
http://example.com/fr
http://example.com/de

Which pattern is the best for SEO?

1. eng. url + lang:

http://example.com/fr/buy
http://example.com/en/buy
http://example.com/de/buy

2. localized url + lang:

http://example.com/fr/acheter
http://example.com/en/buy
http://example.com/de/kaufen

3. localized url with no lang specified

http://example.com/acheter
http://example.com/buy
http://example.com/kaufen