Editing subscriber:firstname (Czech)

Hello! It is possible with a custom shortcode. Here is an example:

add_filter('mailpoet_newsletter_shortcode', 'mailpoet_custom_shortcode', 10, 6);

function mailpoet_custom_shortcode($shortcode, $newsletter, $subscriber, $queue, $newsletter_body, $arguments) {
  // always return the shortcode if it doesn't match your own!
  if ($shortcode !== '[custom:czech_name]') return $shortcode; 
  
  $name = $subscriber->getFirstName();
  
  return $name ? apply_czech_name_ending($name) : 'abonent';
}

The implementation of the apply_czech_name_ending() function which will translate Peter to Petře etc. is left to you, obviously. There are probably libraries for that on the web. Add the resulting code to your functions.php. After that you’ll be able to add the [custom:czech_name] shortcode to your newsletter, and Czech name endings will appear in the content.

Please see the custom shortcode reference for more detail: https://kb.mailpoet.com/article/160-create-a-custom-shortcode