maybe...

Well, I guess that sometimes Watto didn't want anyone to know what he was saying, or he wanted to look better, or say the part where he says to Anakin that if he doesn't stop Qui-Gon's betting, he'll end up owning him too. Perhaps he didn't want Qui-Gon to hear that or Qui-Gon would figure that maybe he was wrong betting on such things and change his mind and Watto would be out of a great gamble.

accuracy

For the same reason that native speakers of any language use that language when speaking to each other -- it's easier and they probably prefer it. It's a pet peeve of mine when characters do the opposite in movies, for example, in Karate Kid 2 when Miyagi and Sato (and the other Okinawan characters as well, for that matter) speak English exclusively, even to each other when no native English speakers are around. (I doubt that everyone on Okinawa speaks English all the time!) A little Japanese-with-English-subtitles sprinkled in would have added a touch of realism.

good point.....but

It does seem unclear why they would speak in Toydarian, but often when people are in uncomfortable situations they speak their native tongue. Annoying- yes, but understandable.

Secrecy between master and slave

When a Watto is giving Anakin, his slave, orders, he might not want anyone else to understand. My parents often talk in French when they don't want me to hear something, since I only speak English.

don't be silly

he aparantly prefers to speak in his native tongue.wouldn't you?

Language Differences

First off, it is not Toydarian that Wattoo is using-it's Huttese. According to the novel version, Huttese is a very strong language, with many profane words with no equivalent elsewhere; just like the Inuit (Eskimo) tongue has over 200 words for snow and Arabic has over 1000 words for sword. When Wattoo wanted to scold Anakin or insult Qui-Gon he would use Huttese, simply because it has more appropriate words to do so.

And Also..

I agree with #21275, that Anakin and Watto were speaking Huttese, because if you recall, Watto's species is Toydarian. That doesn't mean that Watto necessarily spoke Toydarian. And Watto owned Anakin's mom, Shmi Skywalker, before she had Anakin. So, I guess when Anakin was born, Watto taught him Huttese so it would be easier for him to talk to Anakin because Watto doesn't speak Engish that well, as you can see in the movie.

Agreed with Comment 21725 - Huttese, Not Toydarian

Tattooine is controlled by the Hutts, so Huttese is the de facto 'lingua franca' of the planet. It makes perfect business sense for Watto to make sure Anakin speaks Huttese as well as English. There would be no profit in teaching Anakin how to speak Toydarian.