Displaying Special Characters and Letters in Drupal 7 Site
Try installing the languages in which you display the data. To do so go first install the "Languages" module which is already part of the core and then go to admin/config/regional/language and install any language with it.

If it's not in a formal language, try changing encoding by using this article:

https://www.drupal.org/node/8408

Though, they mention in that document that it's problematic to change Drupal's encoding.

If showing these letters is rare, consider displaying these as an image.