[MOSS2007/WSSv3] "The expected version of the product was not found on the system" while installing update

[ISSUE]
I just tried to install the June Cumulative Update on a test environment. After the "Running detection" step, I got the following error:

"The expected version of the product was not found on the system"

[CAUSE]
Some troubleshooting revealed that we had installed SP2 for WSS and MOSS on the environment, but not for the installed language packs. After we installed SP2 for the WSS and MOSS Language Packs, the update installed just fine.