Two options I know.
1) You can use modifier keys to change some characters. E.g., option+u and then A to get Ä
I use Keyboardviewer (kind of hidden on a Mac so do a Spotlight search) to see the common ones.
2) You can select different keyboard layouts in System Preferences. Of course your physical keyboard will have the same labels so you either need to memorize them or you need to buy a Spanish keyboard, or get labels that you can stick on an English keyboard. For the former, go to Apple's online store and pretend you are buying something and you will see what keyboards you can choose. For the latter, here, for example, is one I found for somebody asking about this for Japanese:
FORITO Japanese Keyboard Stickers, Japanese Keyboard Stickers Black Background with White Lettering for Computer (Japanese) - https://www.amazon.com/FORITO-Japanese-Keyboard-Background-Lettering/dp/B07JG4YYWB/ref=psdc_3015407011_t1_B075Q2GP8M