Please understand that we cannot build websites here, per say. there are over 200 themes we can use but none have the feature you indicate. What you ask for is not possible on a free hosted WordPress.com blog. We can set only one interface language here > Settings > General for our blogs.
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.
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.