pre_insert_term filter is only for the term itself and not filtering all $args and the plugin in question had only one opportunity to transliterate slug hooking sanitize_title(), which looks like a last resort instead of expected behavior, but this is not the source of the issue.
The source of trouble is strtolower(), which is only converting ASCII characters 🤦
$name_match = null;
if ( $name_matches ) {
foreach ( $name_matches as $_match ) {
if ( strtolower( $name ) === strtolower( $_match->name ) ) {
$name_match = $_match;
break;
}
}
}