probably the most convenient way would be to click "General Information" the right the language of the string table you are modifying under "General Information\String Tables" and export it. then look at the text file in your project directory (in english it's "String1033.txt"), and make all of your modifications using your favorite text editor. Then import the file the same way you exported it.
Installshield's default error messages are not in the string tables. If so, there wouldn't be a problem at all. You will only find strings from the default dialogs there but no error messages, like for example the ones that are displayed when updating a release.
those strings are defined in some pre-compiled object files in the IS program directory.
In "...\Program Files\Macrovision\IS2008\Script\isrt\Include\SDINT.H" there's an external function declared
external prototype STRING SdLoadString( NUMBER );
which returns a string for each of the defined error codes. you could build a mini script that could extract all of the current string values and write them to a file. then you could modiffy those strings, and re-implement the SdLoadString function.
back stuff up before modifying! do a quick test before throwing too much time away. good luck!
