Adding a subdomain solved the issue.
Here is a guide for other users
If you want to move from subdomain (en.topic1.com) to subfolder (topic1.com/en/) set the redirects with the following code in htaccess (code was provided by Ipstenu (Mika Epstein)).
Lesson #1: ALWAYS put the WP calls at the bottom.
Lesson #2: Always separate your .htaccess sections.
# Subdomains
RewriteCond %{HTTP_HOST} ^en.topic1.com [OR]
RewriteCond %{HTTP_HOST} ^www.en.topic1.com
RewriteRule ^(.*)$ https://topic1.com/en/$1 [R=301,L]
# BEGIN WordPress
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]
# END WordPress
Wait a couple of hours as it did not work immediately.
If you are on the same server as the subdomains (en.topic1.com) was hosted you should have everything redirected.
If you are on a new server you must add a subdomain or the server will not know that for example en.topic1.com exists and will serve a 404
To do so, in cPanel, go to subdomains and create a subdomain pointing to the document root of the folder where the htaccess is. This will be the same folder which contains all the files for topic1.com
It should now work.
As a practical example, if anyone goes to https://en.meetitalia.org/travel-phrases/travel-phrases-1-ciao-hello/ he will be redirected to
https://meetitalia.org/en/travel-phrases/travel-phrases-1-ciao-hello/
remember that you must repeat the above steps for all subdomains you had on your previous installation.
And remember to thank Ipstenu (Mika Epstein) as it was her help that got me this far.