Language based redirect with .htaccess

[bollettino]

Hello, everybody. My forum https://www.hspir.it is in two languages, English and Italian. All English topics are in the directory https://www.hspir.it/en, Italian topics are in https://www.hspir.it/it. I would like that people using an English browser would automatically be redirected to the first folder, Italians to the second. I tried to configure a .htaccess file, but, as soon as I uploaded it, the forum became impossible to open, or I saw a message saying that there were too many redirects. I tried these two kind of codes in .htaccess:

RewriteEngine on

RewriteCond %{HTTP:Accept-Language} (it) [NC]
RewriteRule .* https://www.hspir.it/it [L]

RewriteCond %{HTTP:Accept-Language} (en) [NC]
RewriteRule .* https://www.hspir.it/en [L]

and, alternatively, this one:

RewriteEngine on

RewriteCond %{HTTP:Accept-Language} (it) [NC]
RewriteRule ^$ https://www.hspir.it/it [L]

RewriteCond %{HTTP:Accept-Language} (en) [NC]
RewriteRule ^$ https://www.hspir.it/en [L]

In both cases, the forum doesn?work. After restoring the original .htaccess file, or even without using any .htaccess file, the forum is working again. What am I doing wrong?

I am using vBulletin 5.5.1.