The best way to do this would be to use the builtin functionality of Editor's multilingual support. Just open the *.ism of the repackaged application, or open the *.MSI in Wizard mode to convert it to an *.ism, then:
Installation Designer -> General Information -> Project Properties -> Languages.
then
Releases -> Release Properties -> UI Languages
This will create language-specific *.mst files that are applied when you use the 'Language Selection Dialog' option. They work by containing all the different language string and codepage changes necessary to localize the MSI into a different language.
