Liability primary arises from negligence in professional conduct (not from simple error). If a patient reports that (Maay Maay) "I am bleeding and have terrible pain" means (in English) "I feel great", the translator's negligence is the source of any damage. Since the translator is probably acting within the scope of his/her employment as a translator for the hospital, the hospital ends up bearing the actual liability (unless the translator willfully mistranslated the statement, so was acting recklessly, outside the scope of employment and thus bears the responsibility).
Translators may not be hospital employees, and might be a family members. Nevertheless, this does not take the hospital off the hook, because they are supposed to provide a medical translator. The Joint Commission, which is a major accrediting agency for health care in the US, holds that hospitals are required to provide translation services, meaning a competent medical translator, which summarizes the professional standard of care that the hospital is neglecting. Title VI of the Civil Rights Act of 1964 also prohibits discrimination on the basis of national origin, and most states have requirements to provide translation: see this article for an overview of legal mandates to provide translation services. This page gives a number of links to specific cases, also this page.
So relying on an unqualified translator is negligent.