Created attachment 119206 [details]
example of the problem
when i make Bold a text (in Greek) using the Caladea font it messes the text up. letters of this text overlap or disappear and the text becomes unreadable!
it seems to happen after a letter with a diacritical mark (which the greek language uses). text without diacritical mark doesn't have a problem. see attachment.
In the provided screenshot, it is clear that the version of Caladea you have installed lacks Greek character support, and LibreOffice is falling back to Segoe UI and something that looks like Times.
Unfortunately this isnt something that we can fix as the font doesnt come with greek support from its authors. If they update the font to support greek, we would gladly update the version that we bundle. I'd recommend using any of the other serif bundled fonts that support greek like DejaVu Serif and Liberation Serif.