You can find these validation rules in module system and then in the domain module. However, in 126.96.36.199 you can't edit the rules. Try to add a validation rule for the name attribute in the account object in the administration module. Adding the validation rule seems to be fine. But I didn't tested the actual message in deployment, so you could give it a try.
The account object inherits the system.user object and I didn't tested it, but this validation rule should overwrite the default system.text. Because you can't edit the system.user, you have to edit the administration.account object.
It seems that they are indeed not translatable, can you file a bug report?