If I build a multilanguage routing system like this below and a language switcher where I change the value of the locale variable with a post request, will I have all the views (i.e. mysite.com/en; mysite.com/it; mysite.com/es;) indexed on Google SERP, or only the ones in the default language set as "locale" in config/app.php?
..or only the ones in the default language set as "locale" in config/app.php?
No all of them as long as the googlebot can find and crawl the URL's. If you have a language switcher with actual links google will find them and crawl them.