No caching for language negociation in a subdir: mysite.ca/drupal/en etc?

Hi!

I'm trying to get Boost working on a site whit language negotiation under a directory, something like:

https://mysite.ca/drupal/en
https://mysite.ca/drupal/fr

My first try was to follow all instructions and I obtained a 404 for the entire site (The requested URL /drupal/en was not found on this server.).

Then I saw https://www.drupal.org/node/1459690#comment-7261264 where it is said to put the Boost .htaccess extension in the site root .htaccess The result was that the site works, no more 404 eveywhere...

But no cache is generated. There is nothing in mysite.ca/drupal/cache/normal/mysite.ca

I did verify that the drupal caching is disable letting boost work and the dirs are at 775.

Is this a bug? Or did I forgot something?

Thx

Note: in those issues, I saw that the prefix (e.g./en /fr) problem should have been resolve?
https://www.drupal.org/project/boost/issues/1359042
https://www.drupal.org/node/1151296