SmartyPants only replaces plain ASCII quotes to the configured format, but it cannot detect other types of quotes.
A workaround could be to first dynamically replace the German quotation marks to ASCII ones and then call SmartyPants afterwards:
$someText_withGuillemets = $page->someText()->callback(function ($field) {
$field->value = str_replace(['โ', 'โ'], '"', $field->value);
return $field;
})->kirbyText()->smartypants();