Bingowings said:

It makes sense that Gungans should speak Gungan when talking only to other Gungans but most of the time they are speaking to humans or within their company. It might add to the drama if they sometimes spoke Gungan with humans around to imply a level of secrecy.

I wouldn't go down the route of Magnoliafan though as action orientated high dialogue movies don't lend to too many subtitles.

Lucas was wise not to give Yoda an alien language track as it would have dragged ESB down to a crawl.

i don't think we need subs in other to tell what they are saying, we can simply just tell by their gestures what is going on