The relation of language to thought has puzzled us ever since. Interestingly, however, for some time now--certainly through the twentieth century, and now into the twenty-first--political reformers have operated on the assumption that the direction of causality goes from language to thought, not the other way around. Thus controlling language leads to controlling thought. If people can be trained not to say certain things, they will in time, it is thought (hoped, feared), no longer think those things. The prohibitions can become so deeply ingrained that they become second nature, habitual practice that no longer requires deliberation or conscious self-control. And that, of course, is tantamount to controlling people, which is the ultimate goal of most political reformers.
The point was brilliantly made in George Orwell's Nineteen Eighty-Four, and it is recently explored again by one of today's great essayists, Theodore Dalrymple. (I have my own brief discussion in chapter 7 of my Actual Ethics.)
Although I am no expert on the matter, I tend to think the political reformers are right: controlling speech is a first, and a significant, step toward controlling behavior. Whether language ultimately determines thought or the other way around, I am not sure. But as the range of taboo speech increases, the realm of free exploration of thought shrinks. It is for that reason that I think it must be resisted. Control one's speech for decency, clarity, penetration, perspicacity, wit: but not for political expediency. Refrain from saying what is unintelligent, unbecoming, or ugly: but not what offends bigotry.