Hi, ateeeate -
Am I right, that it might be easier to upgrade to system 10 instead? but that it would also require upgrading to new software - (even more expensive)??
Yes, on both counts.
An OSX install includes multiple language capability by default; extra languages can be removed (or not installed to begin with) by intention, but are included by default.
However, that does not include Classic - the OS 9 install used for Classic remains single-language, and would need appropriate version Language Kits in order to regain that flexability in use. Since the minimum version OS 9 that can be used as Classic is OS 9.1 (OS 9.2.1 and OS 9.2.2 provide better compatibility in the Classic mode), not much has been gained.
Unless, of course, you also replace any software which you need to use in Japanese with native OSX versions. If you do plan on replacing the machine with a newer one in the near future, then that may be money well spent - since newer machines boot only to OSX, and the newest ones which use Intel processors can not use Classic at all, then you would probably need to get such software anyway.
The cheapest (and probably simplest) way to get to where you want to be, using your current hardware, would be to get a retail OS 9.2.1 Install CD and install that OS - either on top of your existing OS 9.0.3 System Folder, or via a Clean Install (note that a Clean Install of OS 9 removes nothing) which would give you a new System Folder in the new OS version.
Article #58176 - Mac OS 8, Mac OS 9: Performing a Clean Installation
Since a retail OS 9.2.1 Install CD includes the ability to install Language Kits, you can do that at the time of installing the new OS itself. To do that, on the page with the Start button, locate and click the Customize button. In the new page, scroll down to Language Kits and checkmark it. To the right of that item a pulldown menu should become active - select Customized Installation from it. In the new window, select the Language Kit(s) you want to install. Okay your way back out to the main installer page, and click the Start button.
This will update (if you are installing on top of your existing OS 9.0.3, it will actually remove the old and install the new) Language Kits on the machine.
Once that is done, download and install the OS 9.2.2 download update - it should have no problem installing on top of the new OS 9.2.1 - be sure to download the language version that matches the native-language version of the new OS 9.2.1.