Seems like the Bulgarian translation file was broken since day one (tracked it back to version 1.0.4). I think the reason is that the file was uploaded in binary instead ASCII mode. Currently using latest version 1.2.7.
Also, unless I add "AddDefaultCharset UTF-8" to Apache's config entries created in Cyrillic are not displayed properly (language in preferences set to English).
On top of these when I import an ICS file with Cyrillic entries they are displayed as question marks on Web Calendar. Might possibly get fixed if the language could be set to Bulgarian in preferences but I don't think it should be a prerequisite to import properly cyrillic characters.