The option Improve Meta Title for Multi-Language Site says that If there are 2 or more active languages, language name will be added to Meta Title.. And yes it is but the name of the language is in English no matter what language is used. So for instance, we have Polish and English, and the word Polish is used on Polish version of the website. I think it should use the translated word for Polish instead. I also noticed the same thing with changing the website language (from footer) -- it also shows the languages only in English no matter what language is currently used.

Could you do this so that the header site displays the language in which the site itself is?

Try this;
<?php
osc_add_filter('meta_title_filter', function($title) {
  return $title . ' - ' . osc_current_user_locale();
});
?>