You said it, the language barriers causes them to limit their options, but most of the Mexicans crossing into the United States are better educated and overall better off than the poorest of the poorest Mexican.
You will be surprised how relatively few Mexicans from the southern states (ie. Chiapas and others along southern Mexico) are actually crossing the border into the US vs. the number of Mexicans from Northern Mexico such as Sonora, Chihuahua, and other north Mexico states, in addition to Mexicans from central Mexico. It just so happens that Mexico's poorest states are overwhelmingly the southern states, with its central and northern states being among the most progressives, not only economically but also in direct government investment in education, health, and the sort.
The average northern and central Mexican is overall better educated, better off than the average southern Mexican, and from those northern and central Mexicans, the one's who are prone to migrate to the United States are those who have, on average, a higher education and economic well being than most within that particular Mexican population.
In addition, there are many non-Mexicans from Central America and other Latin and non-Latin American countries using Mexico as a gateway into the United States as well. In fact, there are complaints from the Mexican authorities of the number of non-Mexicans who use Mexico to enter into the United States, but they are being deported into Mexico even though they are not Mexican and/or lack Mexican citizenship.
Check the data and you will see what I have been stating here. In fact, the magration patterns on a global scale shows that most of the people migrating from the developing world into the developed world are not the poorest of the poor, but those who are in their economically productive years, better educated and better off economically than most.
Much of the data on international migration is readily available, please look into them.
-NAL