There are at least a few reasons to consider transliterating WordPress permalinks. First of all, it is more convenient for international visitors to navigate through readable URL addresses that contain only Latin letters. Latin alphabet is a base for all western languages including English, German, Spanish, Portuguese and French. Some experts claim that romanization of URLs may have some positive effect on SEO and position in search engine results.
Although there are some WordPress plugins that allows to transliterate the WordPress slugs, but mostly they are either limited or no longer supported. Below you can find a few code snippets that would automatically replace the Greek, Bulgarian, Russian and Serbian Cyrillic characters with their Latin equivalent in WordPress permalinks saved with Permalink Manager.
Please note that the code should be pasted to functions.php file located in (child) theme directory. The functions will be applied to all new posts, pages and terms permalinks and if you would like to transliterate the old WordPress permalinks please regenerate the old URLs following the instructions posted here.