I just watched a clip of Thieu's speech in English from the early 70s. He seemed to have a good command of English. His Wikipedia page doesn't have much biographical information on his formal education and he didn't appear to have been educated in the West or have gone to an international/missionary school. I know his son went to Eton College, but that still doesn't explain his own knowledge of the English language which is surprisingly adept and surreally sophisticated for someone who was busy serving in the military and fighting wars most of his life.
His wikipedia entry mentions among other things that:
Thiệu was a son of a small, well-off landowner who earned his living by farming and fishing. [...] His elder brothers raised money so that he could attend the elite schools run by France, who were Vietnam’s colonial rulers. Although not yet a Catholic (he would convert later in life after getting married), Thiệu attended Pellerin, a French-run Catholic school in Huế, the imperial seat of the Nguyễn dynasty.
A bit further down:
With the help of his brother, Nguyễn Văn Hiếu, a Paris-trained lawyer who served in the upper echelons of the State of Vietnam government, Thiệu initially was enrolled in the Merchant Marine Academy.
There also is:
Thiệu transferred to the National Military Academy in Đà Lạt.
Courtesy of TED, there's another passage:
In 1957, and again in 1960, Thiệu was sent to the United States for military training. He studied at the Command and General Staff College at Fort Leavenworth, Kansas, and in weapons training at Fort Bliss, Texas, as well as at the Joint and Combined Planning School of the Pacific Command in Okinawa.
Put another way, he was anything but a random peasant who works rice fields. He was well born, well educated, and well connected.