I'll be honest with you. You sound like you're judging someone else's choices by your own standards, rather than what might be best for them, and what they consider to be their priorities.
Moreover, you come across as a bit of a snob who perhaps has judged the woman by superficial qualities. She might be much more intelligent than you think but just doesn't care about the things you care about, enough to extend a conversation about matters she considers uninteresting.
Now, it's entirely possible that someone who marries into your wife's "famous" family will have certain duties and expectations, like appearing gracious before the press. In that case I would suggest you focus on her perceived ability to perform those duties rather than her perceived intellectual capacity.
If not, then the only questions that matter are, "Does she make your brother-in-law happy?" and, "Does he think she embodies the qualities that make a good wife?"
As for your in-laws asking what you think, I would have suggested you tell them you don't feel comfortable talking behind their son's back, but if he would like to have an open discussion about it, then you're happy to tell him your opinion of her -- with the understanding that, in the end, it's his opinion that matters and that you will be happy for him no matter what.
(Edit) To clarify the statements in the last paragraph. You should not go behind your brother-in-law's back to discuss your opinion with his parents. If asked, your answer should be something like, "I don't think I should talk about this without (brother-in-law's name) present".
If, instead, the entire family wants to have a discussion as a group, and if your brother-in-law indicates he would be open to everyone's honest opinion, and if you all agree that, in the end, it's his decision to make -- then you can and should express your opinion freely.
As you might expect, saying, "I don't think she's very smart," can come across as a bit rude. Instead take a more tactful approach, and ask leading questions:
I'm concerned that you don't share the same interests. What do you guys do together? What do you talk about when you're alone?
Do you ever feel like she's holding you back in any way?
Does she seem comfortable when you both are together with your own friends?
Do you think she's willing to learn Spanish, if only to feel like she can speak freely with your family? If not, do you think that will be a problem?
These are fair questions. When you marry someone, you often marry their whole family, plus all of their friends. If one partner doesn't get along with that extended group, it can cause a friction that grows increasingly uncomfortable over time.
However, you should not exaggerate those differences, or focus on any one particular trait. Perhaps, to him, her most important quality is her sweet disposition, or her style, or simply the fact that they have great chemistry.
Moreover, you don't know your brother-in-law's intended fiancée the way he does. It's possible that her outward congeniality masks hidden depths, which (for whatever reason) she doesn't feel comfortable showing to just anyone. You have to first check that you're trying to keep an open mind about her.
Listen to his answers to those questions, without prejudgment. Be willing to change your opinions about her, and her "fitness" for your brother-in-law. Frame your own opinion in the context of what might conflict with what he wants, or which might cause significant problems in the family. Always try to make it about him and his values rather than about you and your values.
As an example: My wife doesn't much care about politics or current events. She has little interest in watching the news or discussing social trends. When my close friends first met her, this came across as shallow and slightly conceited, since we love to talk about those things. Moreover her overall appearance, her general "niceness", and certain cute mannerisms, make some people judge her as having little intellectual depth.
Of course, I knew her differently, and knew that she could and would talk about all kinds of things that interested her, in great detail and depth. Over time, my friends and family have come to know her as well, and come to love her for who she is and not who they expect her to be. In return, my wife opened up and started to express more of an interest in the things my friends value -- although she still rarely reads the news, she cares about what's happening in the world.
There are all kinds of reasons to get married. Some of us want a partner who reinforces who we are. Some of us want a partner who, instead, forces us to be someone else. You can't really know unless you are the two people involved -- and because that's impossible, it's unwise to jump to quick conclusions.
