You will go through customs in JFK-NY.
You usually pass through passport control, get your luggage, and then go through customs. You get a customs form that you fill out while on the plane.
If you have something funny on your form, then might ask to see inside your luggage. Otherwise, you pass right through.
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I'm not sure, but they will probably have a Spanish interpreter, since it is somewhat a common language in the U.S.
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There are signs (in English) when you get off the plane, and there is only one way to go, at either airport, so you cannot get lost. When you switch plans at JFK, you might need to go to another terminal to get your connecting flight (after you pass through customs).
In Reagan-National, the luggage pick-up is downstairs if you are in the new terminal. If you get off the old terminal, luggage pick-up is by the door as you go out. They have signs, so it is hard to miss.