I have a bit of hard time trying to understand what is wrong with the 3rd line of this rewrite rule :
Options +FollowSymLinks
RewriteEngine On
RewriteRule ^(.*)\.html$ $1.php [L]
RewriteRule ^fr/lecon-de-kite\.html$ kite-lessons.php?lang=fr [L]
RewriteRule ^fr$ index.php?lang=fr [L]
RewriteRule ^it$ index.php?lang=it [L]
when I try to reach http://localhost/kitemeup/fr/lecon-de-kite.html I get an Not Found error
Have it like this in your site root .htaccess:
Options +FollowSymLinks
RewriteEngine On
RewriteRule ^fr/lecon-de-kite\.html$ kite-lessons.php?lang=fr [L]
RewriteRule ^(it|fr)/?$ index.php?lang=$1 [L,QSA]
RewriteRule ^(.+)\.html$ $1.php [L,NC]
Just understand that ordering of rules is important as mod_rewrite processes them from top to bottom. Your html -> php rule was taking precedence and converting all .html URIs to .php making your 2nd rule ineffective.