In some languages is “ů,ž,š,č,ř” commonly used and incompatible font is problematic.
Other option is give option to change font.
Excellent point, @Tolomaj, and welcome to the community! 
Just to understand things more clearly, when you see the incompatible characters are you typing directly into the notecard or copying from elsewhere and pasting text into the notecard?
I write characters directly on the keyboard.
Hi, @Tolomaj.
I am actively working to gather up-to-date information to share. Thank you for being so patient. 
Hello @Tolomaj
First of all, thank you so much for your feedback.
At this point we have not received many requests to support multiple languages. We would be very happy to add this to our roadmap when we receive more inquiries from our members.