Does anyone know (like in tried and succeeded) if I can have D2009 both in English and in French?
It's not so much to generate French or English applications but to have the IDE available in either language to take screen shots and make demos in French or in English.
What would be involved: complementary install plus some shortcut param switch?
Install one version in a virtual machine. I hear VMWare works really good.
Nope, won't work. If they were different versions (ie. one 2007, one 2009) then you should be fine, but if they are the same version you'll need to go the VM route as Jim suggested.
Why would you want that? If you need internationalized versions of your program, you should better use at tool for that purpose (e.g. dxGetText) rather than recompile it with a different Delphi version. In that case I definitely suggest using the English IDE, so you can translate the RTL errors from English to the target language.