Monodevelop on OS X and Displaying UTF-8
Since this is a duplicate, which still has not been closed, and the issues in question have been fixed, I will quote the accepted answer there:
Several common Mac programs have clipboard manipulation features that seem to corrupt MD's memory. It's a known issue and is documented here.
Unfortunately my answer remains that same as my answer in the thread you linked: the Mac version of the Pango library that MonoDevelop uses for font rendering does not support glyph fallbacks, but you may be able to work around this by using a font that contains all the glyphs you need. If you could be more specific about the charsets and where you are seeing issues, we may be able to provide more specific answers.
[NOTE: both these issues have since been fixed]