Hey @ysugimura. Thanks a bunch for reporting this! We've actually seen more reports of people seeing this with Shift JIS: https://github.com/owncloud/client/issues/6291 and https://github.com/owncloud/client/issues/4960
NSIS (the tool used to build our current Windows installers) does not fully support some encodings everywhere on the installer. Some versions in the past (e.g. 2.2.4) managed to get it working properly, as seen in https://github.com/owncloud/client/issues/6291#issuecomment-362215719. But current versions might be affected by this encoding issue.
The upcoming 2.5 client release will replace NSIS by MSI as the installer building tool and this was actually, one of the reasons for the switch. You can help us get as many strings translated into Japanese by joining the project
I'm sorry for the inconvenience