The answer is the USB to PS/2 adapter. The input device has to be specifically designed to support both USB and PS/2 modes for this adapter to be of use. You can't just hook any USB keyboard up to it and expect it to work

Thanks @maxtherabbit, any documentation with a little more detail so I can understand a little more the problem?

Again, thanks a lot!

USB keyboards use a protocol to communicate over USB port. Some USB keyboards, also support PS/2 protocol (completely different from the one used for USB), which is used to communicate over PS/2 ports, when using the adapter. The adapter is completely passive, all it does is connect the PS/2 port wires to the correct ones on the USB keyboards's port, nothing more. When the keyboard supports both communication protocols, it will detect upon connection which one should use an go with that. Unfortunately, it's not really predictable if a random keyboard (applies to mice too) will support or not both protocols, however most new keyboards are USB-only. Your best bet would be getting a keyboard known to support both (oldie Microsoft keyboards come to mind, just like Logitech ones).

That picture gives me anxiety

Thanks @aha2940 that actually answered my question!.

Yeah, I already ordered a PS/2 keyboard, finding an International English AT keyboard sounds like too much effort ATM.