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. There is no FTP access to free hosted WordPress.COM blogs and we cannot install individual plugins into them
There is no upgrade that you can purchase from WordPress.com that provides FTP access and the ability to install plugins in a free hosted WordPress.com blog. Read this to understand the differences > http://support.wordpress.com/com-vs-org/
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.