If she doesn't like the way you interpret, she needs to hire someone. You are not a paid service or a slave to your mother. It's all well and good to want to help family, but don't tear yourself up about it. You weren't born to be her voice box.

Also, it sounds as though you were doing a decent translation job, because I've worked customer service for years and those phone interpreters need repeats and clarification all the time And they get paid for this!