Different plugin files for each site of a multisite web

Hi,
I have a plugin problem on a WP multisite.

I run two sites, each in a different language. I installed MailChimp Widget plugin to get a subscribe form to my newsletter mail-list. It is shown on the right sidebar and works fine on the both sites unless I want set one language to the sidebar on the main site and a different language to the secondary site.

What I need is to get one subscribe form in English and another one in Slovak. The plugin enables to customize the “send” button, confirmation and error message in the widget menu which is different for each site. However, inscriptions “Last name” and “First name” can be adjusted only at the level of the plugin file itself, which is just one for the both sites. The same holds for an additional sentence (“Would you like to receive your newsletter?”) that I also need in two language versions.

I tried to duplicate the whole plugin, put it into a new dictionary (“wp-content/plugins/mailchimp-widget-2”) and set a different plugin to each site. (WP really found two different plugins, with the same name.) However, this solution would require some additional adjustment in the plugin files I was not able to carry out.

I would be very thankful for a suggestion how to set different plugin files for each site or for any other idea how to solve my problem.

The web is: http://www.labanbratislava.sk