I'm sorry but that's not possible on a free hosted WordPress.com blog. We cannot only set one interface language here > Settings > general and there are no themes we can use that provide what you want.
There are code restrictions here at WordPress.com. No iframes and no JavaScript can be used. http://en.support.wordpress.com/code/
There is no FTP access and we cannot install plugins because this is a multiuser blogging platform ie. our blogs are not free standing and there are security issues that would arise if that access were allowed. Google translation widgets are Javascript and it's stripped out to preserve security on this mutiuser blogging platform. http://en.support.wordpress.com/code/
I have translation on my WordPress.com blogs. There are two workarounds and you will find them here > http://onecoolsitebloggingtips.com/2011/02/14/translation-widget-for-wordpress-com-blogs/
If those do not suit you, you can write a blog post in the two languages and then post the two versions of the entry on the same blog, make each language category, create a custom menu and add the categories pages to it.
http://en.support.wordpress.com/menus/#creating-a-custom-menu
http://en.support.wordpress.com/menus/#adding-category-pages-to-your-menu