I like the idea of the Gungans speaking Gunganese to each other, and English to the humans. It would also make dubbing a lot easier since I wouldn't have to write English dialogue around the lip movement in those parts, and could just string together some Gungish/Gunganese jibberish words that match the lip movements. And as stated, it would imply a level of mistrust between the Gungans and Humans. I suppose that whether or not subtitles would be used in those parts would depend on how well the scenes read without knowing what the Gungans are saying.