Is possible just assign a language for a group member, then another language for another group mebmers. Then each group only can see or use the language assigned. 

Group member of Spanish will don´t need to know or see the website is available for english language users, group member of enlish don´t need to know or see the website is available for spanish language users. 

If a group member is assigned with 2 languages, then they can see the language toggle option.

Not sure I understand, but take a look at this:

It will display forums based on user language. Forums app only.