I have set up my WP as multisite (with the definition in wp-config.php and the commands in .htaccess files) and it seems like everything is working fine in the admin of the two sites. The problem is that that main site, running directly under the main domain is redirected to the subdomain (holding the secondary site), in the browser.
For example, the main is running under mydomain.com and the secondary site (for a second language of the site) is running under en.mydomain.com.
I have followed the instructions here: https://codex.wordpress.org/Create_A_Network (setting it up as sub-domains, of course)
I also installed the following plugins: Polylang, Multisite Language Switcher, Wordpress MU Domain Mapping
What is missing in my configurations and/or what am I doing wrong??