I was unable to upload images to an album in a locale different than the default. Not sure if this is a correct fix but i had to change this function in ImageGalleryManager.php in order for it to work:

   public function UploadLink()
   {
      return parent::UploadLink()."?album=".$this->getSelectedAlbumID();
   }

to this:

   public function UploadLink()
   {
      return Controller::join_links( $this->BaseLink(), 'upload?album=' . $this->getSelectedAlbumID() );
   }

Could not find much info about it so maybe someone finds it useful.