I have a way to do it, but it isn’t the most ideal one, because it’s not DRY.
if ( 'nl' == $key ) {
add_filter( 'acf/settings/current_language', function() {
return 'nl';
} );
} elseif ( 'en' == $key ) {
add_filter( 'acf/settings/current_language', function() {
return 'en';
} );
} else {
add_filter( 'acf/settings/current_language', function () {
global $sitepress;
return $sitepress->get_default_language();
} );
}