As for the gibberish from M&L I mentioned earlier, someone transliterated one of Mario's PiT phrases as something like "api-para-ta-toro-tu". I'm pretty sure all of the SS and PiT ones are available for download here.
When he sais so, it sounds like the Italian "ma chi te l'ha detto?" (but who has said this to you?)
Anyway those are non sense words...XD
Mario and Luigi aren't doubed normally and, instead of making him talking in long dialogs, N. prefears using the nonsense dialogues when they talk, which can sound like "Italian+English", even if what they are saying doesn't mean anything...
Maybe, in the Mario & Luigi RPG serie, Nintendo wanted to give more personality to Mario and Luigi (because they are the protagonists and main heroes) making them like "talking" with their typical (and cute-funny) accent, while the other characters make only different sounds.