I looked around with Google and within Drupal's site, yet I'm surprised nobody seems to have reported this. Maybe I haven't looked hard enough?
Clean URL's work, the i18n module works, and the gallery module with its "clean URL's" works fine too. I have the English, French, and Japanese languages enabled. Everything is fine... almost.
If you go in the album from another page, since the link to the album is rewritten properly, you'll get the page in the proper language. However, clicking on any picture will bring you back to the English version, since the link is not rewritten. If you write the address manually to get a specific language and to go in a specific sub-album, you'll get the correct language, but with the front page of the album.
To summarize:
(Note: "album" is an alias for the default "gallery" path.)
/album -> Front page of Gallery in English.
/fr/album -> Front page of Gallery in French. (Links point back to default language, English.)
/album/meetup -> "Meetup" album page of Gallery in English.
/fr/album/meetup -> Front page of Gallery in French. (Like /fr/album .)
I tried separating the gallery_embed_uri in the $conf['i18n_variables'] array of the settings.php file. This rewrites the URL's properly. However, when going to /fr/album/meetup, I have the same issue.
I tried rewriting the .htaccess file in the Drupal root folder, which is rewritten by the gallery module in order to make its clean URL system work. I replaced all instances similar to this: