Hello,
Advanced Installer (10.9.1) currently supports adding custom dictionaries for the MSI installer (for languages not supported out-of-the-box). However, these dictionaries do not apply to the bootstrapper. Based on the answer here, it is currently not possible to add new language support to the bootstrapper.
Please consider providing a way for us to add new translation dictionaries to the bootstrapper itself. When our customers selects a language for the installer, they're going to expect the entire install process to be in that language, including the status of the prerequisite install. (We're only using the Basic UI with Progress interface for now, so it's really only a few status strings, but it would still look more professional to our customers if all languages available to them were supported by the bootstrapper.)