For general information on panels see Panels.
Headlines on all kind of installer panels can be translated in the following manner:
Create a custom translations file for each language, for instance german:
and add it as resource to the installation descriptor:
A special use case are translations depending on conditions. Language packs don't "know" conditioned translations. You can workaround this by defining one and the same panel for different conditions with a separate panel ID:
The translations are divided now depending on the panel ID for the appropriate panel: