Mystery solved.:)
I watched the network tab in Chrome dev. tools to see what's happening when you drop-down that list.
It definitely calls the system/country/getlist processor and in the code of that processor is this line that gets the countries:
include $this->modx->getOption('core_path').'lexicon/country/en.inc.php';
In that file, though, there's a later line:
$_country_lang["sc"] = 'Sint Maarten (Dutch part)'; // Sint Maarten (Dutch part)
That's overwriting the value for Seychelles.
It should be:
$_country_lang["sx"] = 'Sint Maarten (Dutch part)'; // Sint Maarten (Dutch part)
Make that change, clear the cache, and you should be good to go.
That mistake is in all the country files. Could you please report that here:
https://github.com/modxcms/revolution/issues