I can see the points made by all of you but -maybe it's the in me- I still feel something is left out of the picture.
I think the purposes of the organization do not necessarily match those of the people who attend those congregations.
I can see why they can open Kingdom Halls to attract old American expatriates. After all, large numbers of witnesses are not young anymore. You go where these expatriates don't feel at home (because, indeed, they aren't at home) and get them at a time when they need company and people "like them". That is understandable.
I can also understand that a native speaker of English might want to hang out in a congregation where she can understand what is being said. Fine.
But think, not as an American or Englishman moving to another country, but as a local. You realize that English is the de facto world language, yes, but it also plays the role Latin plays in the Catholic Church. If you want to be able to communicate with the guys who really run the show, you need to speak English. And it cannot be the sort of English that you'd use at a cafeteria, but serious jargon. You have to know words such as "rapture", and clichés such as "the faithful and discreet slave", which you can only acquire after some time and in a congregation where such terms are used all the time. So, a smart local brother will understand this; and so will smart people coming from abroad.
I can think of someone moving from the US, Canada or any English speaking country for the purpose of being recognized as a "hard worker" where "the need is greatest", taking advantage of money exchange differences, finding a wife (not necessarily a local, but another missionary from home, say) and then being in a congregation while he gets to learn the language. This would not be what I would call "the corporate reason to open an English-language congregation", but I am sure the Watchtower wouldn't miss its "side benefits".
Since you mention Mexico and Central America, it is of interest to me that many American Jehovah's witnesses are of Latino descent, yet those aren't the ones moving back to the countries their parents migrated from. Whatever Americans think of them, most Latinos who have spent a long time in the US, or were born there, do not have a real command of Spanish, but they are way closer to achieving that than an American of a different descent. Why is it not the Latinos that move down? They could also be the ones preaching to expatriates. English they do speak well.