I'm primarily looking for a GIS data source for postal codes in Japan as well as census data for different scales of geography. Has anyone found a good source of these data sets that is in English?
Not sure if this is what you had in mind, but there is a CSV file available from the Japan Post web site that has postcodes and their corresponding locations in Romaji (roman characters or English if you wish to think of it in those terms) for all of Japan. It is available from a link on this page: http://www.post.japanpost.jp/zipcode/dl/roman-zip.html There is no equivalent English page (that I could find), but if you can identify the kanji '全国一括' (also identifiable by size:(988,276Byte)) it's in the lower right corner of the group of links (the data is provided by prefecture (ken) as well, thus the large number of links), and the file is zipped as: KEN_ALL_ROME.LZH
There are some other attributes included in the file (not LAT/LONG), but I did not research what they are.
If this is enough as is, or not what you wanted, that's fine. However, if you are interested in knowing more about the data set, I may be willing to put a little more effort into looking into it, if asked.
At this point, I am not sure about Census data sets. Sorry.
Naturally, the polygons (shapefile) are independent of language, but you can use the post office file to match the attributes with the romanized data. The shapefile for postal code areas is PAREA-Zip, which is sold by PAREA (a division of KOKUSAI KOGYO CO.,LTD.), and whose website is http://www.parea.jp/datebase/area_map/#zip There is an email provided on that page (no English page), and they will likely reply to a succinctly written English message if you are interested in purchasing their data. For census data, the GIS formats are available from PASCO (presumably not free) in their PFM series (probably PFM25000-P). Their English page describing these datasets is http://www.pasco.co.jp/eng/products/map/