Iphones: What to do to use an iphone 6 to text in Spanish?

Now, I have to do word by word because the phone gives me an English word in place of the Spanish word I type. For example, if I type nietos, it gives me Nietzsche.

Maybe set your keyboard language to Spanish

You have to change the Keyboard language to Spanish.
It should be in Settings>General>Keyboard>Keyboards>Add a new keyboard. Select Spanish.

Once you've done that, you should see a globe icon near the lower left corner. Tap and hold to switch between English (assuming that is your default) and Spanish.