Hey Marco,
Thanks for writing in to us. Using the plugin on a site to present two separate languages is actually pretty complicated. It might be possible to use a third-party multi-language WordPress plugin to set different active languages for different pages in your site. Another option would be to have two separate WordPress installations for the site, one for the main site and one for the /en/ subdirectory–with each installation having the plugin installed to it. If either case could be used to get our plugin to display messages in the desired languages per page, there would still be the issue of translating the field names in the plugin.
In this case, you may actually be best served using the MailChimp provided embedded forms on the two separate pages of your site (as it looks like you began working with on the main Spanish site), overriding CSS as necessary on your site and manually changing the displayed text for each of the fields per language’s signup page. That would let you have the two forms display similarly with different text. Alternately, you could have the plugin on one site and an embed form as you currently do, and have the list itself configured with the field names matching the language you’re using the plugin form, then try to customize the embed form on the other language’s page with manually changed field names.
If you have questions about this, please let us know.
-Gabe
