Let me answer your second question first regarding language…

The front end (and most of the backend), can be easily translated by changing the text in one file.  The file is named "mkt_language.php".   Changes to this file, will propagate to all areas of your front end, and most of the backend.

As far as integrating the theme into your exisiting theme, you may be better off starting with this theme — and then modifying it to be more of what you like.

Or…

You can install a new WordPress installation of Market into a sub-folder named "Online Store" or something like that, and then just place a link in your main site's navigation area.  Then, all you'll need to do is change the colors of Market to match your site, and upload your site's logo.