Combine the coriander and cumin in a small pan, set the pan over medium-low heat, and cook, stirring and tossing often, until the spices are very fragrant and the coriander seeds turn a shade or two darker, about 8 minutes. Let the spices cool slightly and pound them in a granite mortar (or grind them in a spice grinder) to a coarse powder. Combine the spice powder, condensed milk, lemongrass, galangal, turmeric, sugar, salt, pepper, and all but a few tablespoons of the coconut milk in a blender. Blend until smooth, then pour the marinade into a container, preferably a deep, narrow one. Swish the remaining coconut milk in the blender and pour it into the container.