I believe FormIt uses the cultureKey value in the system settings to include the desired country file (default us.countries.inc.php).
These files are stored in: core/components/formit/includes/
There is a dutch lexicon (nl.states.inc.php) present, but it shows a list of states. If you want the country list in Dutch, I guess you have to translate the us.countries.inc.php and name it nl.countries.inc.php in order to get it called by FormIt based on the cultureKey.
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There is such a list in core/lexicon/country, you might be able to access that directly, or at least copy it to the formit lexicon.
I see the code and i found:
$country = $this->getOption('country','us');
then: [[!FormItCountryOptions? &selected=`[[!+fi.country]]` &country=`[[++cultureKey]]` ]]
in the documentation i don't found country param but is used!