Longkong was the name given them at Viva Fruits (which also sells longans, visible at the bottom of the first photo), but langsat is the most common of many vernacular names. Two principal varieties, and numerous named cultivars, further confuse the issue.
Consistently, though, the juicy segments inside each of these longkong (150 baht per kilo) had the texture of a firm ripe grape and a flavor suggesting an ideal grapefruit. Usually there was a little fiber to pull free from the center and a few small seeds to work around, but my only real gripe was the thin rind, which pulled away easily but tended to get caught up under my fingernails. A thicker-skinned fruit called loku (or is it just another variety of longkong?) provided a happy solution to that problem; details to follow when I catch up on my photos from Singapore.
Aw Taw Gaw market, stall 5/12, Bangkok