Comment 11 for bug 503759

cablop: what is happening here is/was that the Thai fonts package was "stealing" (claiming) a generic font name, which happens to be the generic font name ("Microsoft Sans Serif") that Mono specifically requests.

If there are any other font packages installed with request the same substitution then you will see the same results with those.

What is happening is that (an unknown) font is requesting the diversion of /everything/ to itself, but then it doesn't have the glyph coverage in order to cover the diversion, and so boxes are displayed, that is still happening for some reason with CJK. Bug #539008, comment #12 on the other bug report notes this.