Watto the Toydarian is indeed a racial stereotype (but not the one you're thinking of...)
According to Andy Secombe (the actor who portrays Watto), his character is openly intended to be based on a stereotypical shady Italian salesman.
Q Did you ‘invent’ Watto’s voice all by yourself?
AS: I did! When I got a call there was an audition for loads and loads of voices. I auditioned and asked what they were looking for! I was told that George had an idea of the second hand car salesman. So, I came up with (talks like Watto) “Hey, how yer doing”; that kind of voice. I don’t know why I came up with that, it was the first thing that came out.
Q. It sounds a bit like the Godfather to me!
AS: Yeah…I guess it does. Well it was actually based a bit on (laughs)…there’s an English actor called Michael Ripper. You know the Hammer Horror movies? He was always like “O you don’t wanna go up there!”. It was sort of half based on that character.
Q. There’s a story on the internet that it [Watto] was based on Alec Guinness’
role as Fagin in Oliver Twist.
AS: No, he isn’t Jewish! Watto is an Italian... Toydarian! No way he was based on that character.
Andy Secombe interview | www.StarWarsInterviews.com
Gunray's accent was unashamedly Thai.
Lucas apparently had Silan Carson pre-record the lines in his own voice, then brought in a considerable number of voice actors to overdub the lines. When he found an accent he liked, he then had Carson learn that accent and re-dub the lines over again.
What was it like recording Nute Gunray's voice for Episode I? It's quite a specific dialect that the character uses--was your direction just as specific?
George and Rick listened to all kinds of different actors from different countries voicing the words that I had spoken on the set, and eventually they decided on a Thai accent. It was very strange, because I could see all the scenes that I was in, but with this Thai actor speaking the lines I had spoken, trying to do my intonations with a heavy Thai accent--and then I had to learn his accent and re-record it. It was the most bizarre process.
I was recording for a couple of days, and it was so much fun, because in the room were just myself, George and Robin Gurland, and we just went through all these lines. Doing voice recordings is great fun--you've got this great big screen and you see the whole thing, and you're trying to lip-synch it. But you've got to hang around while people are chopping and cutting the tapes, so there's a lot of messing about to be had--and we had it.
Silas Carson: Hero with a Thousand Faces - Starwars.com
Jar Jar speaks like a baby because his script was partially written by a toddler.
Jar Jar's language and speech patterns are a mixture of "pidgin English from the Samoan islands and Pacific islands and the Caribbean" mixed in with some baby talk words and sounds that Lucas's infant son Jett would use (notably the word Gungan is his son's word for "car" and the name "Jar-Jar" was what the boy named the character when he saw an initial sketch)
Especially jarring to Best were the charges that his character was a racist stereotype. To this day, these charges confuse Best rather than anger him. According to Lucas, Jar Jar’s speech patterns (“Meesa called Jar Jar Binks. Meesa your humble servant”) were to some degree based on “pidgin English from the Samoan islands and Pacific islands and the Caribbean,” but “it was a completely made-up language.” In fact, Lucas says, some of his son Jett’s baby talk even ended up as words. (The term “Gungan” is what Jett used to call cars.)
Entertainment Weekly - In defense of Jar Jar Binks
The name Jar Jar was assigned, as was Gungan, which came from Lucas’
young son Jett’s name for tractors and trucks. “I just liked the sound
of it,” says Lucas. “I’m always on the lookout for
interesting-sounding words. I have to come up with hundreds of them,
and I don’t like names with x’s and z’s in them that people like to
use in space films.”
Jar Jar Binks: A Digital Star Is Born - RollingStone.com