I doubt any agency would have bothered hiring a sign interpreter. Lol
They work as independent contractors. Their primary job is usually with state or province but they work on an hourly wage for others who hire them. All the agency would need to do is hire them to follow the lady around and function as her driver for her shift. They get paid as a driver and a negotiable rate for their time engaged in sign interpreting services.
I have also worked with foreign language interpreters. Some of them do it as a second job. Usually the primary job is working in a court and the secondary jobs are for what attorneys need them to interpret and translate in sworn deposition testimony and sometimes in other proceedings. It's the same thing with sign language interpreters but there is fewer of them. They get sworn in to translate or interpret truthfully just like the testifying witness.
Funny story but I once needed to hire an interpreter of a somewhat rare Eastern European language to depose an old lady who slipped and fell on an allegedly slippery sidewalk at my client's pharmacy. It was a snowy day and I couldn't understand why she walked through the snowy parking lot and entrance to the store rather than drive to the drive through window since she was driven to the pharmacy by her husband who was likely driving very slowly. There was no reason to get out of the car. So I asked her if she was aware of the drive through window. She said she was. And then asked her why she didn't use it. And she gave this very long answer in her native language. Probably a 2 minute answer. So the interpreter says, translating her answer, "yeah I thought about doing that, but I wanted to show the pharmacist my medicine so I brought it into the store with me."
And I am sitting there saying to myself....WTF?????! All that was 1 sentence????? Unfortunately this happens at translated depositions and not much you can do about it. I never got an answer to my question that made one iota of sense. Sometimes that is a good thing if you actually go to trial and don't settle the case. The strategy there would be leave the nonsense answer on the record and let her attorney try and get her to correct it. Which he never did.