The best way, apparently, is to define a custom language pack and override all the entries that you want to change, but there is an error in izpack (5.0-Beta and even rc1) that prevents overriding custom language packs from the packages found in the preinstalled language of the installer in this In case you need to change the language pack that is included in the created installer banner (just unzip it, find the language pack, change the file and write it back), or you can download the izpack source code and change the language pa chum there, so an installer is created, it will contain a modified language pack.
Language packs
located in: izpack / izpack-core / src / main / resources / com / izforge / izpack / bin / langpacks / installer
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