Langsat is one of my family's favourite fruit. Its peel is thin and brown and releases white sap when cooked (or even when you are peeling it to get to the fruit). Like the longan, the langsat's sweet milky translucent flesh encases its seed. Unlike the logan, each langsat fruit has many soft seeds which taste really bitter. At the markets, be careful to distinguish the langsat from the variety called "duku", which has a thick skin and flesh that is more sour.
Malaysian langsat trees supposedly only bear fruit twice a year: in June and July, and in December and January; so when they are in season, my father and I can each polish off about a kilo at one go, just like durian.
Langsats are perishable and spoil after 3-4 days at room temperature. It can be difficult to find langsats that are at their freshest, and that are clean and have not got alive / dead caterpillars on them. So far the freshest and best-tasting langsats we have bought were from a fruit stall in Toa Payoh near Courts, which also sells durian.