There’s two different issues here: Should you interpret at a non-Catholic church? Should you interpret at a non-Catholic church where there is a history of anti-Catholicism?
Question 1: If this is a job for which you are compensated, substituting for your friend is not problematic. It would be similar to a nurse logging the doctor’s orders. The nurse is not responsible for what the doctor decides, only for accurately entering his orders. If you’re a volunteer, however, this is more akin to agreeing with the message to the extent of wanting to make it understandable to the congregation. In other words, you are participating in helping the pastor to teach his congregation. As non-Catholic churches disagree with Catholicism to a greater or lesser degree, you are volunteering to help teach some degree of error. If the church is not anti-Catholic, it would be all right to accept a paid job to interpret, if non-Catholic interpreters are unavailable. It would be problematic to volunteer to do so.
Question 2: If you know the church actively teaches anti-Catholicism, it would be problematic to accept even a paid job as a sign language interpreter because you would be in a position of having to interpret anti-Catholic remarks and thus participate in teaching anti-Catholicism to the congregation.
