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.
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.