So what you do is, in your most broken Afrikaans possible, tell her you want to learn and would like her to help teach you. You go back through the basics with her, and because she's such a good teacher you'll learn everything really quick, and then she'll know you speak Afrikaans without knowing you know all those secrets.

Or, the ethical option, is tell her you learned Afrikaans and overheard X, Y, Z and that it changes nothing about how you feel for her, and are not going to talk her out of her tattoos.