Cannot find them in 2015-05-04. Do I need a mod? Are they for TFTD? Are we talking about the terrain combo in the New Battle config screen?
Those strings appear when there is more than one terrain available for selection for each mission. The vanilla Alien & XCom Bases, Terror Mission and Cydonia missions only have 1 terrain assigned, so the box for terrain selection won't appear on New Battle. But if you ran the Terrain Pack, for instance, there are several terrains available for Terror Sites.
The strings for STR_FOREST, STR_DESERT, etc., don't also exist on the original game. They were added since in New Battle it is possible to choose the terrain for each UFO mission.