ah, but just to be complete about this, let's not forget to mention here that the YAM (as opposed to the sweet potato) is indigenous to Africa, and pretty much brought over with African slaves to the New World, and to some extent Europe. Though not as much to the Middle East, even though the African slave trade was much larger and longer, the East Africans apparently didn't have as many yams.
back to on-topic matters...
besides the use of Arcane Mark, many Mages, with Super-High Intelligence, oft have many extra languages, some which might be obscure and archaic--they could be using some super odd dialect of Draconian (to use a d20 mainstream example) or, IMC, an obscure branch of Gaelic...another idea rarely if ever mentioned in AD&D (though I think Gygax made a passing reference to this at some point) is the use of a Cypher or Cipher, in other words, a code language. FWIW, there are actual historical instances of "mages" using them, that is if you consider Dr. John Dee, court astrologer to Elizabeth I, and Edward Kelley mages...