While that's somewhat true (although that fails to explain how Sen. Patty Murray (D-WA) speaks) we're not consciously directing the evolution of our language, so it is not correct to say that language is designed. I'm not a linguist, but my understanding nevertheless is that the most popular mechanism for linguistic evolution is quite a bit different from biological evolution.
I always thought market forces (trading) had something to do with it. That and getting your butt kicked by the enemy. No one will ever explain Msss. Murray.
Language changes much as the economy does: in some part from market forces, and in some part from centralized direction.