Grocery stores often have international sections, you may be able to find some stuff you're familiar with in the Hispanic Foods section. There is dulce de leche, but it's not the same thing. You can find canned yerba mate tea from the Guaiaki brand. Again, it's not the one you drink, but it's at least something.
I think this is an exaggeration(Like saying a Brit can't understand an American or something). The meaning of some words may be different but to say they can't understand each other is a far stretch. Some countries still use the vosotros conjugation but I don't think that is going to be an issue.
In my experience, latinos in school often end up congregating with other latinos and they're usually able to get past the linguistic differences fairly quickly (and the linguistic idiosyncrasies becomes a recurring topic of conversation), so I think the concern about difficulty to communicate doesn't apply as much for the native spanish speaker (since they can just switch to english to speak to a non-native-spanish speaker)
Though it does remind me of the time a friend and I wandered into a Billy Connolly (the Scottish comedian) stand-up performance in Dublin, and we both spent the first 45 minutes not being able to understand a single word the guy said. Which was doubly frustrating since the Irish audience around us was laughing their asses off.