The basics they'll actually need
Spain runs on 220V power, so guests need an adapter, not a converter, for most modern electronics. The euro is the currency, and most venues and restaurants accept major credit cards, though small village shops often prefer cash.
Mallorca's high-season sun is stronger than it looks by early afternoon; remind guests attending an outdoor daytime ceremony to plan for shade and hydration, especially if they're not used to Mediterranean heat.
Getting around
Many wedding venues sit down narrow rural roads unfamiliar to first-time renters. If you're not arranging shared shuttles, encourage guests to book a rental car with GPS in advance rather than relying on cell signal alone in more remote areas.
The pace of things
Spain, and Mallorca especially, moves at a different pace than a lot of American guests expect. Restaurants often don't start serving dinner until 8 or 8:30pm, and that's completely normal. Build that into how you frame your welcome dinner timing so no one shows up to an empty restaurant at 6.