Easternese is the language spoken in Tai-tastigon and—given it's name—likely commonly spoken elsewhere in the Eastern Lands. If that is the case, it likely came to be such by being the language of the Old Empire.
It is related enough to Southron that Jame, who speaks Easternese, can understand some Southron words from their resemblance to Easternese cognates, but is still distant enough that Jame can barely understand the soldiers who accost her in Hurlen.
a type of talismanic statue; sometimes translated as "luck-bringer." It's unclear if Res aB'tyrr is Easternese or some other language, but even if it's not, b'tyrr is at least a loan word to Easternese.
↑Dark of the Moon, "Chapter 9: The Haunted Palace" — " 'My lady calls me Gricki.' Jame repeated the name with distaste. It was uncomfortably close to the Easternese word for excrement. […] From the way she spat out the nickname, Jame knew that it meant the same thing in Southron as in Easternese."
↑Dark of the Moon, "Chapter 14: Gathering Forces" — "the noise settled into a chant, one Southron word repeated over and over. Jame recognized it from its Easternese cognate. 'Highness!' the soldiers were shouting. 'Highness!' "
↑Dark of the Moon, "Chapter 14: Gathering Forces" — "a few soldiers did accost them in Southron, which Jame barely understood, with intentions all too clear;"
↑Seeker's Mask, "Part VII: Wilden", III — " 'The pattern of cuts is called kuth. It's used in the public execution of child-killers.' It might, Jame thought, have been the last thing which Bane himself had felt on the Tastigon Mercy Seat under the flayer's knife, unjustly accused of Dally's murder. 'In the Eastern Lands, it would be considered just punishment for the assassination of Aerulan,' "