The Rio Grande Valley region, comprised of the small cities of McAllen, Edinburg, and Mission, Texas, is home to the largest proportion of residents in the United States with limited ability to speak English. Thirty-four percent of residents there—along the border of Mexico—are not proficient in English.
El Centro, Calif., also on the Mexican border, falls into second place, with nearly 32 percent of its residents not proficient in English. The region with the third highest proportion—and located on the Mexican border as well—is El Paso, Texas, where 30.4 percent of residents are not proficient.
Thanks to some great mapping work by the National Journal, which pulled together U.S. Census data, and information from the National Center for Education Statistics and the Migration Policy Institute, we can clearly see where the nation's English-language learners are concentrated and what their primary languages are.
Naturally, those metropolitan areas that border Mexico, as well as cities that have been long-standing immigrant gateways are home to the largest numbers of English-learners. For me though, it's instructive to see the numbers for lesser-known immigrant communities. For example, more than 10 percent of the population in the Washington, D.C. metro area where I live has limited English skills, while three smaller metro regions in Washington state are also home to quite sizeable ELL populations.
Thanks to @JohnSegota at TESOL for tipping me off to this map and its companion infographics. It will be a useful tool.