Why is it that when I’m typing using the Cyrillic alphabet, if I turn on italics, the letters change?

Cyrillic italic originates in cursive. Greek, Cyrillic, and Latin cursive all look quite different from vanilla print versions of the alphabets; it’s only in Cyrillic that the print italic version of the alphabet is based on cursive. (You could argue that Latin italic is based on handwriting, but not on Cursive as we understand it. And given that Latin cursive ended up in German as Sütterlin, that’s just as well.)

Added bonus: there is regional variation in Cyrillic cursives, and hence Cyrillic italics; г italicised looks completely different in Russian and Serbian/Macedonian (Macedonian alphabet). This is a problem for Unicode Cyrillic: you need to specify the language explicitly to get the right forms.

How did the languages in countries like Papua New Guinea (which has the most languages) get verified and who does it?

The main game in town for documenting languages in PNG are SIL International  . I think they do Lexicostatistics to establish what counts as a language. There is the potential for error, as most data is gathered by missionaries rather than academic linguists; but any differentiation between dialect and language is leaky anyway. From memory, the sources did routinely identify dialects within the claimed languages; so it wasn’t a case of lazily calling every village a different language, and I don’t remember the languages looking particularly close. (Unless they’re Oceanic languages, of course, but that’s a later migration, and not the majority of PNG languages.)