htaccess replace all slash (two and more) by one slash - regex

Question

I want to modify an .htaccess file to do 2 things:
Force the addition of a slash at the end of each URL
Replace the excess slashes (2 and more) by 1 single slash.
Here is a set of URLs and their expected result:
https://www.mywebsite.io/fr > https://www.mywebsite.io/fr/
https://www.mywebsite.io/fr/register > https://www.mywebsite.io/fr/register/
https://www.mywebsite.io/fr/register// > https://www.mywebsite.io/fr/register/
https://www.mywebsite.io/fr////register > https://www.mywebsite.io/fr/register/
https://www.mywebsite.io/fr/register///other////// > https://www.mywebsite.io/fr/register/other/
I manage to do the first step which is to force a final / but I cannot replace the extra /.
Can you help me with this problem?
Here is the content of my .htaccess file
Options +FollowSymlinks
RewriteEngine On
## Force HTTPS
RewriteCond %{HTTPS} !on
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI}/$1 [L,R]
## Force slash
RewriteCond %{REQUEST_URI} /+[^\.]+$
RewriteRule ^(.+[^/])$ %{REQUEST_URI}/ [L,R]
## Remove repeated slashs
#RewriteCond %{REQUEST_URI} ^(.*)/{2,}(.*)$ [N]
#RewriteRule (.*) %1 [R,L]
RewriteRule (.*)/{2,}(.*) $1/$2 [N]
RewriteRule ^(fr|en){1}(/?)$ content/index.php?lang=$1 [L]
RewriteRule ^(fr|en){1}(/){1}(registration){1}(/?)$ content/registration.php?lang=$1 [L]

Answer 1

Have it like this: (see comments inline)
Options +FollowSymlinks
RewriteEngine On
## add https and trailing slash in same rule
RewriteCond %{HTTPS} !on [OR]
RewriteCond %{REQUEST_URI} !/$
RewriteRule ^(.*?)/?$ https://%{HTTP_HOST}/$1/ [R=301,L,NE]
## Remove repeated slashs
RewriteCond %{THE_REQUEST} \s[^?]*//
RewriteRule ^.*$ /$0 [R=301,L,NE]
RewriteRule ^(fr|en)/?$ content/index.php?lang=$1 [L,QSA]
RewriteRule ^(fr|en)/registration/?$ content/registration.php?lang=$1 [L,QSA]

Answer 2

With your shown samples, could you please try following. I have also fixed your small issues of missing flags too in your question. Please make sure you clear your browser cache before testing your URLs.
Options +FollowSymlinks
RewriteEngine On
## Force HTTPS
RewriteCond %{HTTPS} !on
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI}/$1 [L,R]
## make slashes to 1 at last or uri, in between slashes will be automatically moved to single slash.
RewriteRule ^([^/]*)(?:/+)$ %{REQUEST_URI}/ [L,R=301]
RewriteRule ^(fr|en)/?$ content/index.php?lang=$1 [NC,L]
RewriteRule ^(fr|en)/(registration)(/?)$ content/$2.php?lang=$1 [NC,L]