The Spanish-speaking congregations are driving all the growth in the US. If it were not for them, US growth rate would be in the solidly negative numbers. More than 1/4 of the JWs in the US belong to Spanish-speaking congregations.
I can't answer about other countries, but here is my impression on why the Spanish-speaking congregations are doing so well here.
In general, the most growth comes from areas where there are large populations of recent immigrants. In areas where there are not so many recent immigrants, Spanish-speaking congregations suffer the same malaise as any other congregation.
In areas where there is a large recent-immigrant population, those immigrants generally come from very very poor circumstances. They most often come to the US for work, so they can send money back to their poverty-stricken families. In fact, it is common for an immigrant worker to send the majority of the money he earns back to his family. That's why there will often be 10 or 12 guys living in a one-bedroom apartment.
But of course that's not much of a life, either. Plus these guys are often lonely, with their entire social structure ripped away. Plus they're likely missing the family they left behind.
Plus they've likely been raised to have respect for the Bible and/or organized religion, or at least a fear of God. Atheism is not common in Latino countries.
Now, with all that in mind, imagine you are an immigrant worker enjoying your one day of rest from your 70 or 80 hour workweek. Here come Jose and Rosa, who are clean and well dressed and smiling. They have Bibles, and talk about God, and offer nice shiny literature in your own language (maybe the first thing you've read in your own language for months). They invite you to their Kingdom Hall, even offering to pick you up, and you agree to go.
When you get there, what do you see? Everybody is well dressed and clean. Everybody is smiling and laughing and look happy. Everybody comes up and introduces themselves to you and shakes your hand. The meeting starts, and people are quiet and look in their Bibles and seem very reverent.
This is all very impressive and very appealing. As you continue to attend, the loneliness begins to dissipate, and the social structure you left behind is replaced. Before you know it, you're studying the questions to be baptized, and you go out recruiting yourself.
In some Spanish congregations, a zealous pioneer can have 10 or 12 studies easily. At DCs, it is common for several hundred to be baptized, and at Circuit events, you might have near a hundred baptized. These numbers dwarf anything done in English language congregations.