This plugin is from Outsource Online, and I found it through the Joomla extensions directory listing.
It got nice reviews, and it didn't need core file hacking, which is a big plus. So I went ahead, downloaded it and installed it in both my and one of my client's website. The plugin gives you a box like shown below, and I can already report that the language changes to Dutch on Dutch driven joomla sites:
On my client's website, it worked straight out of the box. Installed the plugin, enabled it, and we were good to go. On my own site it was a bit more work, as this site uses a RocketTheme template with template overrides for lots of pages, including the contact page.
Luckily, that is resolved easily. I just opened the template override php file and placed the code for the captcha image there where I wanted it. Then disabled the auto-add to contact us form in the plugin and my site was good to go, too.
Great extension!