The search features do not work when using non-English characters like Japanese in the Finder application and other web mapping applications in ArcGIS Online.
Unable to reproduce using the following test app:
http://jsapi.maps.arcgis.com/apps/Viewer/index.html?appid=cc478ba960b14511a76da16c80d4a583
Run the app and enter 札幌市 into the search box and you'll see that a valid feature is returned.