Finnish is already supported in this plugin, but you need to set it as a main language in your WordPress (in wp-admin page Settings / General, field Site Language).
It should be translated, I see it in *.po file, please check Site Language field on the page Settings / General in your wp-admin, it should be Finnish, because the plugin uses this option to show messages. Thanks!
I appreciate you taking me for a fool, but obviously I already have the site set as Finnish. Either the .po file is broken or something else is conflicting.
It also does not matter if the browser language is set to Finnish. I have tried that as well.
Please refer me to a site example where this is working so I can see it, otherwise I have no reason to believe this is working correctly.