The files with the .msg extension are text catalogs per language, used by
InstallJammer. 

Those ones override the definitions found in the standard InstallJammer 
installation. This is the standard way to change the InstallJammer texts 
without changing the original files into the InstallJammer installation: copy
them next to the project file (.mpi). 

The original definitions are located into:

INSTALLJAMMER/lib/msgs/installer/*.msg
INSTALLJAMMER/Themes/Modern_Wizard/*.msg

The files have been copied from the InstallJammer 1.2.15 version.

Take into account that if you change any text attribute of a panel into the
InstallJammer app, that value will override the one located into the files of
the current folder. The problem is that this way in the project the value
will override only the value for the English language.

So if you have already done that, edit the mpi file manually, look for the
entry with the text you had changed, and remove the entry. This way the 
value will be loaded again from the files in the current folder.