How do search engines treat multilingual sites where the language is stored in cookies?

Question

Search Engine Optimization

Hello.
I am developing a website on the international domain with the possibility of simple localization into other languages ​​in the future. I would like to know how search engines treat sites whose language is determined dynamically (by browser language or, say, by IP) and then stored in Cookies. In this case, the user will be able to change the language of the site himself in his basement. A good example of such a site is VKontakte. I know that there are smarter ways to set the language:

• Buy a domain, such as site.ru and site.de
  
• Make a subdomain, for example ru.site.pw and de.site.pw
  
• Add language to all links, such as site.pw/ru/ and site.pw/de/
  

But I heard that search engines define the first two options as different sites, and the third option is more difficult to implement in the sense that all links will have to specify the language through a function, for example (I didn’t think of it better):

<a href="/<?php current_language(); ?>/prices"></a>

But about the implementation method through Cookies, which I'm asking about, I couldn't google anything sensible. Share your opinions and experiences, please.

Answer 1

IMHO, they don't care what's in the cookies. Yandex and Google recommend doing this:
https://yandex.ru/blog/webmaster/15326
googlerussiablog.blogspot.ae/2013/04/hreflang-upda...