Sounds like you're making a small amount of progress.
Since the blog is installed in a folder that is its own application (aka application starting point) in IIS, BlogEngine.VirtualPath should be:
<add key="BlogEngine.VirtualPath" value="~/"/>
The only time it's needed to set the virtual path to something such as "~/blog/es/" would be if the blog folder wasn't its own separate application.
The error currently on your site is:
'/blog/es/blog/es/themes/Standard/site.master' does not exist.
If you change BlogEngine.VirtualPath to "~/" (as shown above), this particular error should go away as BE will then look for your site.master file in the correct location:
After switching BlogEngine.VirtualPath to "~/", it sounds like your blog comes up, but none of the CSS styles are applied? One possible cause for this is a setting in the control panel. In the control panel, under Settings -> Advanced, there
is a setting called "Trim stylesheets". Try unchecking that and saving.
Right now, this is the only thing that comes to mind. If your blog still continues to not have styles after unchecking "Trim stylesheets", try leaving it alone, and I'll navigate to the site and see if I see anything. Of course you're welcome
to try anything you feel might help.