The exchange rate between US$ and Quetzales has remained fairly constant for years. That’s actually a regional exception. Two Central American countries “gave up” and use the US$ as their currency — Panama and El Salvador.
The Bank of Guatemala provides a page when you can check the current exchange rate or get historical information. You can find it here. While the default currency is USD, it has options for lots of others.
If you intend to do transfers from European banks, you will need an IBAN for your account in Guatemala. Again, the Bank of Guatemala comes to the rescue with this page. Even if you think you have the IBAN, use the verify portion of this page to check what you have.
Note that if you transfer to a Quetzal account, the money you send will automatically be converted. Various banks have different rates with, in my experience, Banco Rural being the worst.