I'll tell you.
JWs have been so excited about preaching to English-spearkers and making the field 'grow', that they have not been studiying with only those whos first language is English. Many of them, if not most, have been taking anyone with any level of English as a study.
Many Mexicans that return from the States to their country know Spanish very well. It should be left to the Spanish congregations to take care of them, even though they might be fluent in English. It hasn't been the case. But now that is changing.
In the last circuit assembly, the COs gave a talk to all the Mexico Inglés congregatoins about the new instructions. And during the week of the CO visit, a video was shown to all the Inglés congregations the importance of people to study the Bible in their mother tongue because it "speaks to their heart" and produces better results.
JWs in the Mexico Inglés field didn't think it would radically change things in their territory. Due to the new instructions, they are now skipping entire neighborhoods. They are even doing away with their records and taking a brand new census showing where people who's first language is English reside.
There are 170 English congregations in Mexico. About the same number there was in the 2012 service year.