OK, it probably goes without saying but Japanese family registries are, of course, written in Japanese. Though some larger city halls may have an international relations division, it and any other municipal offices will generally not undertake the task of translating family registries.
The method of maintaining or updating the registry's contents may vary from city to city. The older koseki records I have seen consist of handwritten notations entered by the city official who was notified of the given event (for example births, transfer of honseki-chi, marriage). Because parts or even the entire record may be handwritten, deciphering it may be a bit of a challenge. A newer copy of a koseki I have seen employs the same basic format but seems to have been typed on a computer or word processor.
If you're interested in seeing the basic layouts of koseki, have a peek here.