If I remember correctly VB6 and earlier can be easily decompiled... it's been a while since I've seen something written in VB6! As mentioned above the default is very likely stored somewhere else. You will need to figure that out and change it there. Good luck
VB6 offered compilation as true binary executable etc or the older type of executable similar to vb5 and older where the executable is basically just a sort of interpreted code that gets further processed by the runtime.
You can get IDA Pro (or another good dissasembler) and dissasemble the application (not the installer) and see where the variable which holds the language selection gets initialized to a default, or hack the function which reads the selected language from registry or a configuration file ... for example the application may have something like this
Public Sub LoadDefaultSettings()
dim Languages(0 to 1) as String
Dim Language as Integer
Languages(0) = English
Languages(1) = French
Language = 0
Call LoadTextFromFile(Language) ; read the texts from a text file based on the language value selected
[...]
End Sub
If you're smart enough, you can dissasemble the application, find out where this "Language = 0" is, or where the "loadtextfromfile" function/procedure is called and figure out what bits change in the executable if you change that 0 to 1, or you replace the register where the language variable is stored with a constant (1 or what you want)
If you're lucky, it's just a matter of changing 2-3 bytes in the executable... but it's not changing a "0" to "1"... it's more like changing bits in 2-3 bytes ...