[Nyororin]

Have you tried just saying to them 日本語で結構です?

I`ve never really encountered this problem - anyone I speak to in Japanese speaks back to me in Japanese, so I can`t really give you a real answer as to why it happens. However, if someone *starts* talking to me in English, when their English ability is obviously much much much lower than my Japanese ability, they will often try to continue on in halting English even after I answer them in Japanese. That`s when I tell them that Japanese is fine.

I`m wondering if the people you are talking with are people who also are involved in the same field? (translation, etc) They may be trying to make use of their very limited chances to speak in English with a native speaker.

Or... Your accent, etc, could just sound really bad to them - so they`re trying to be nice by making things easier for you.