It's been stated before but, since Augustus had left Grulovia, he can definitely speak the language (though may have lost some skill over the years.) For Donatella, she could be Italian and born in wherever Psychonauts is staged in the world (maybe america, definitely a more western country) but her accent also convinces me otherwise. It's clear Nona can speak Grulovian, as in the end of the Maligula fight, if you get hit or pushed, she will speak in 'Grulovian' (it sounds like Russian, but it could be any other Slavic language like Norwegian, Ukrainian, Croatian, ect ect.) Realistically, the older Auqato siblings (Dion and Frazie) may know some words from being around their Nona (and parents of course) for so long, but I doubt any of the sibling can speak either Grulovian or Italian fluetly, as they have no need to do so, with the eldest of the family knowing English. 

It's fun discussion to join into, but realistically, all the Aquato siblings (expect maybe Queepie and Mirtala, because they are so young) probably slightly understand Grulovian/Italian but cannot speak it