A great amount of the growth in foreign language congregations in the USA is due to native English speakers learning the new language and changing over to help satisfy the need for elders/MS/pioneers. Another significant percentage is due to immigration. And then there is also the birth rate among immigrants to the USA. Frankly, there is still some growth from the field but it is negligible. Finally, because of ingrained American prejudice and the fact that the vast majority of elders in congregations of all languages in the USA are English-speaking natives, non-native publishers aren't held to nearly as high a standard as born-in Americans are. It's a lot easier to remain a JW if you're allowed to go through life as a half-assed JW.
So the Spanish (and other foreign language) growth in the USA is much like everything else in Watchtower world. It is only partly real and the rest is illusory.