Well it depends, in Japanese as they pronounce the "r" as "L" or "d" somewhat at times, in Japanese its something like tell-ah or tear-dah. In english if they use regular pronunciations its probably tear-rah. If you say it slow it sounds weird so say it a bit faster and you might get it =P
Sorry, but that's not entirely correct. It's the other way around: "L" is pronounced more like "R" because there is no "L" sound in Japanese. In Japanese I would expect they would pronounce Terra as "Té-rah" like Sandslash said. Also, the "r" would be pronounced trilled, which means it sounds like the "d"s in "ladder". In English it'll likely be pronounced with the normal English "r".