Can I install an App from the Spain App Store (US Store is my default)? I'm in Spain for a few months and I have an Orange prepaid SIM card, and I'd like to install Orange's "mi Orange" app for checking my balance, usage, etc. The app is only available in the Spanish App Store. Can I install it?
No, the iTunes account are country bounded. So you can't buy a Spanish app with your US account. 

But if you'll be in Spain for a few months, I assume you have a Spanish address? So there is no harm in creating a new account for your few months in Spain.
All you need is an address or a gift card from Spain.