Hey, just to throw this out there I am someone who advocates a single language. However, I believe that should be for entry into the US for citizenship. There are also different reasons for why I believe in that.
However to play devils advocate, Sen. Jack Murphy (R) and may be using this tactic for another motive. This kind of change is not usually advocated by the liberals as alluded to above, but by the conservatives. Mainly because those who don't speak English well are typically poor and from societies/countries that are a bit more progressive or socialist than ours. As such, they typically do not vote for the conservatives. So, by creating an artificial bar through the language barrier, it can block out certain people from voting.
Sure this instance is about driving, although as others have pointed out the signs over the roads are international symbols and no language or even literacy proficiency is needed. Otherwise, tourists visiting from other countries would never be able to drive here and American tourists would never be able to drive in other countries if neither could follow the road signs correctly. So by putting a bar on getting a drivers license, it allows a crack in the door to put a bar somewhere else from a government issued item. Another to note, when people get their drivers license is when they can also register to vote. There are other places where people can register, but the most common is the DMV or the equivalent in your state. By blocking out non English speaking people from getting a license that indirectly stops many from registering to vote.
For me it's a toss up issue. I think the enforcement of English speaking for citizenship or a Visa is a must. Then the natural progression would trickle down and make this a moot point for enforcing English for a drivers license. The problem I have with this decision is that it's assbackwards. It's arbitrary and done for the wrong motives in my opinion.