You will need to choose the right font that supports your language (which I am not sure about) –
https://fonts.google.com/?subset=greek-ext
https://fonts.google.com/?subset=cyrillic-ext
and then use the same. You also need to import the font manually in your child theme style.css file using @import where you specify the URL that include subset attribute in the URL as seen above. The theme by default does not include these languages due to performance reasons.
Once decided on the font, you can choose the chosen font in the theme options Fonts tab so that the headings or body font is changed to one of your choice.
Hope this helps.