Recommendation / best practice to create multi language site

Hi,

We are currently using GeneratePress (GP) in one language (English) but we have a new need to incorporate content in a second language (Spanish) to our site. The requirements are:

1) Main site pages will be translated in both languages
2) Some posts will be available in both languages
3) Some posts will be available in only one language (could be English or Spanish)
4) We will have an EN/ESP option in the navigation menu to switch between languages
5) When a user switches between languages, the site should navigate to the same content page/post in the new selected language when the translated content exist 2)
6) When a user switches between languages, the site should navigate to the main page if the current content requested doesn't exist in the new selected language 3)

Could you please recommend suggest what best practice to incorporate multi-language to our GP site with the above requirements?

Please specify also if GP doesn't support any of the 1-6 requirements.

Thank you very much for the help