According to Wikipedia, the Treaty of Amity and Commerce between Siam and the United States (or Roberts Treaty of 1833) was written in Thai and English, together with translations in Portuguese and Chinese annexed to it. Why was it felt the need of annexing a Portuguese translation to it?
The Portuguese were the first westerners to reside in (then) Siam (now) Thailand. The Portuguese embassy is the oldest embassy in the kingdom, and now a monument. Do mind that the relations go back 500 years. As such they enjoyed, at that time, a special status. By the time this treaty was signed, it was purely honorary. By then Portugal was no longer an important power.
Portugal had trading relations in Southeast Asia going back to the 16th century. The main Portuguese outpost was Macau, off the coast of China, but Siam was also an important "port of call."
Either the Thais or the Americans (or both) may have felt more comfortable in Portuguese than in the other party's language.
Courtesy of the Library of Congress, here are some excerpts from a transcript of the 1833 treaty: