While Japan indeed has only one national language, in practice it has many different dialects of that language which are not very mutually intelligible. I live

While Japan indeed has only one national language, in practice it has many different dialects of that language which are not very mutually intelligible. I live here, any my girlfriend says she can't understand lots of people from various more-remote places. She can't even understand people from the opposite side of her own prefecture. So people frequently have to switch to the standard "Tokyo dialect" when talking to people from other parts of the country. Of course, this is probably lessening over time with mass communications, similar to how Americans largely speak with a "midwestern" or "Chicago" dialect used commonly on TV.