Comment 2 for bug 297515

At a glance, I would say that those fonts lack some glyphs and that's why Inkscape uses squares as a substitution. This usually happens with fonts that haven't been designed to work on Latin texts, like asian or arabic typographies, or symbol fonts. Basically any font which lacks one of the characters shown on the correct examples in the list (look at Impact for example).

I can't test it because I don't have the fonts. Could you test if "Latha" or "Mangal" has the 'A' or 'a' characters? What version are you using?