"Which of my fonts can set Bulgarian?" is answered in the Languages section of the sidebar: pick a language and the list keeps the fonts that have every character a text in it needs. The languages are those of the fonts in the collection, so the list itself says what the collection can and cannot do.

The knowledge is fontconfig's, the same as a fc-list :lang=bg on the command line: it holds one orthography per language and compares it with the characters of each font. The section is therefore only shown on systems that have fontconfig, which means Linux and the other Unices.