Sabin's Japanese name is a bit odd, but from what I've read, it's kind of a poor translation of the Katakana which was poorly utilized on the Japanese part. I've read his name is suppose to be Matthew and "Mash" is suppose to be the shorten "Matt" but it's really hard to say Matthew in Japanese without changing some words around and when it was translated it comes up as Mash. My question is where does Sabin come from? I mean Locke and Kefka's names were slightly altered, Terra's was changed to fit the intention, and Cyan's name was changed to fit the limited character restrictions, but where do we go from Mash to Sabin? I mean at least Terra and Tina are not a huge leap from each other compared to Mash and Sabin.
For the subject at hand, Terra is her official name in English, Tina in Japanese, unlike say the Aeris/Aerith debate, Terra's name has been consistent with new media whereas Aeris only appears in the blind idiot translation that is the original FFVII, so Terra is official outside of Japan.
Bartz is official for FFV, even the translation of "Butz" was debatable from the beginning in the fan-translation community.