Hi there,
I was looking for the exact same feature actually. We built an online store for our Spanish customer, and the referral program by Swell met our requirements. I thought I could manually translate the fields but did not manage to do so, sadly.
As a desperate solution after trying other programs, I contacted them and they are actually releasing the translation this evening or tomorrow. My problem is solved, and I hope you will see this message and get your customers "contentos".
Mucha suerte,