EN 
FR If your Gimp, Glimpse or other drawing software displays squares or rectangles instead of the expected text, here is the solution!
How to solve your Gimp or Glimpse font problem ? easy ! You may need to rebuild the font cache. Fortunately, it’s not too difficult:
Open Terminal (if it’s not already open)
Remove the existing font cache:rm -rf ~/.cache/fontconfig
Rebuild the cache:sudo fc-cache -r -v
This should resolve the issue for you 👍🏻
Source : https://askubuntu.com/questions/1397208/font-displayed-as-squares-in-ubuntu-20-04