Through what can the substitution of characters go?

Question

CMS

I got a site from a previous developer on a self-written CMS.
When displaying content on the site, all characters of the Russian alphabet similar to Latin ones are replaced, while the text in the admin panel itself is normal.
I checked the templates, I did not find the components of the system of regular expressions or string processing on the output.
Where else could there potentially be a substitution?

Answer 1

Well, many reasons. You don't say so right away.
It might even be a virus.
Recently there was an article on the topic.