In the US, federal laws require organizations to provide services in the language of the patient if they receive any federal funds. This applies even to solo practitioners. Hospitals and health plans are coming under increasing pressure and scrutiny to ensure they are in compliance with Title VI guidelines.
Beyond the legal requirements, there are powerful economic reasons as well. Using unqualified interpreters can result in significant malpractice risk. A mistake in interpretation can lead to grave consequences in patient outcomes. The bottom line is that it makes sense to work with qualified, professional interpreters.