Configuring a non-standard keyboard

Is there a way of configuring a hardware keyboard, defining the characters associated with each key?

My decidedly non-standard Compaq KB-9963 keyboard (connected via PS2 to USB converter) is "recognised" by my Mac and functions in principle fine, but none of the standard keyboard layouts correctly reflect the actual layout. This isn't a problem for the main character keys, which are in standard QWERTY layout. But the punctuation marks, brackets and special characters are completely mixed up.

The keyboard is in principle designed to be easily switchable between something like 9 different languages and features German, Scandinavian, French, Spanish, Portuguese, Italian and even Greek characters represented in different colours on the keys - a bit confusing to look at at first, but really useful if you regularly write documents in several different languages.

As an example, in French setting, the two principal lines should be as follows:

qwertyuiopà^

asdfghjkléè+

In German setting, this should become:

qwertyuiopü^

asdfghjklöä+

There are similarities with Swiss keyboard layout, but it's really not the same. For what it's worth, the "numbers on the back" of the keyboard are as follows:

Model No: KB-9963

Assy P/N: 122659-108

Spares P/N: 164996-101

It's more than 15 years old but works fine and it'd be a shame to give it up as I haven't found anything similar on the market. Any help much appreciated!