Why did the Roberts Treaty of 1833 have a Portuguese translation annexed to it?

Answer 1

"One original is written in Siamese, the other in English; but as the Siamese are ignorant of English, and the Americans of Siamese, a Portuguese and a Chinese translation are annexed, to serve as testimony to the contents of the Treaty. The writing is of the same tenor and date in all the languages aforesaid."

Source: https://th.usembassy.gov/our-relationship/policy-history/io/