I think to have found a simple solution for this problem using wordpress shortcode.
1- create your gallery post/page in the default Polylang language and insert into it the media you want. Create the gallery and obtain the gallery shortcode (typically like this: [gallery]).
2- save your post/page and then go to the language box (on the top-right) and click on "add new" just right the other language you want to copy your post. This will open a new post/page creating form. Now you can see on language box the original ID of the previous post you have inserted your gallery; copy it.
3- put inside the post (in HTML view) the following shortcode: [gallery id="DEFAULT_POST_ID"], where DEFAULT_POST_ID is the ID you copied from the original post wherein the gallery is.
4- save and repeat for the other languages you need the post/page copied in.
This is the WordPress common method for copying a gallery from a post to another. I just used it inside our preferred language plugin.
Thank you Chouby for this useful plugin!