Your editorial "Let's Listen" (Aug. 8) is right on. The Reagan Administration has acted illegally and without even a modicum of common sense. A great democratic power such as the U.S. has no right to interfere in the internal affairs of another nation. The Administration has, and is acting more in the tradition of a despotic power than one based on free thinking. On the other hand, if Nicaragua chooses to invade its neighbors, the U.S. must act to protect the sovereignty of those nations. The Central American problem is not complicated, it just takes clear thinking, not ideological rigidity.