Comment 3 for bug 1052399

Note that anything starting with "_:" is an itstool placeholder element for content that's (possibly) in another msgid (or possibly untranslated). It's sort of like "%s" in a C format string, and it's not specific to Mallard. One thing you could probably do, if you know the PO file is from itstool, is make sure the msgstr has the same placeholders as the msgid.