Foreign alphabet stripped

It seems every editor I try will replace various foreign language characters from my text. I’m using Serbian Latin alphabet and various characters š, đ, č will be stripped out and replaced with a question mark. (Let’s see if that happens here). If those characters were in the “special character” section I’d be okay, but they’re not. Any way I can adapt foliopress to do this for me?

https://wordpress.org/plugins/foliopress-wysiwyg/