Mexicans have the worst English-speaking abilities of all immigrant groups: Long-term assimilation aside, immigration creates the immediate problem in America of people not being able to communicate with each other. This is not true in all cases: some immigrants speak English better than native-born American do. So which immigrant groups reduce the problem, and which groups add to it?
The General Social Survey's measure of IQ is based on a vocabulary test, and for immigrants it serves as an indication of their English-speaking abilities. The following shows the mean score for each immigrant group:
Mean English-speaking ability (expressed like IQ):
Native-born Americans 99.3
All immigrants 92.1
Puerto Ricans 80.5
English, Welsh, Scottish, and Irish immigrants speak English better than Americans do. On the other end, Puerto Ricans and Mexicans have, by far, the lowest scores, and since they are also large groups, they make a big contribution to the Tower of Babel problem.
Let me give a personal example of the situation: On my way home from teaching night class, there is only one grocery store that is along my route home. All of the sections and the food in the store are labeled in Spanish, and only occasionally do the managers bother to also label items in English. Most of the stock is geared toward Mexican tastes, and even though the store is large, it does not always carry what I am looking for, and it's not like I'm hunting for exotic items. The problem is much worse for whites and blacks who do not understand Spanish like I do. I can't imagine that folks don't complain, but there it is.
Adults schools are an example of how this language problem costs taxpayers. In my city, there are a number of campuses that offer free courses in English to all immigrants. They get to learn the language of success, and I get to pay for it.
Update: Joe Guzzardi writes about his 20 years of experience with immigrants who don't want to learn English at the adult school where he has been a teacher.