Here's how to install STS and translate it into Japanese.
Download STS at Spring Tool Suite Downloads.
Unzip "spring-tool-suite-3.9.x.RELEASE-e4.8.x-win32-x86_64.zip" with standard Zip. (Because the path becomes long when decompressing and it fails, decompress it directly under C)
Place the unzipped STS folder wherever you like.
Check if the English STS starts normally, and close the STS with the "x" button on the upper right.
Download pleiades-win.zip from the Japanese localization file Pleiades Plugin Download.
Unzip "pleiades-win.zip" with standard Zip.
Start pleiades-win setup.exe, select "STS.exe", and press the "Japanese localization button".
Start STS.exe and check that it is in Japanese.
Finally, click the "Like" button.