Hopefully you would never need to hire an interpreter for visiting a police station in Japan. However, in case you get arrested you might need one.
For visiting a foreigner at a japanese police station you need an interpreter. Family members are allowed to visit for 15 mins. This is called 'Menkai' (面会) in Japanese. It is forbidden to talk about legal content at this meeting. Therefore, the police needs an interpreter to check the content.
You can also send letters to the police station. The content of each letter will be checked, so an English written letter needs to be translated into Japanese. An airplane ticket to Japan can be very expensive, so sometimes it is better to write letters.
Further, you find English speaking lawyers in the Tokyo area. Usually, bilingual lawyers are more expensive. It can be difficult to find bilingual lawyers in the countryside. If your lawyer does not speak fluent English, you can always hire an interpreter to help you.
I realized that you can not find many Information on this topic. What kind of information would be useful to be available in English language?