No, you should not install the WorldScript Update (version 1.0) contained in the Apple Extras folder. That update installs older versions of WorldScript software than are installed by the Japanese Language Kit 1.2.
The WorldScript Update v1.0 installs version 7.5.1 of WorldScript I, WorldScript II, and InputBackSupport. The Japanese Language Kit 1.2 installs version 7.5.1 of WorldScript II and version 7.5.2 of InputBackSupport.
If you have installed WorldScript Update 1.0 after installing the Japanese Language Kit 1.2, you should reinstall the Japanese Language Kit 1.2.
Note: System 7.5 Update 2.0, which updates the system to System 7.5.3, includes version 7.5.3 of InputBackSupport. If you install System 7.5 Update 2.0, you should not install WorldScript Update v1.0 on the Power Macintosh 7500 or 9500 computers because it will overwrite the newer InputBackSupport.