From Fedora Project Wiki

The Fedora Localization Project - Graphical User Interface

This is a list of the most important (essential) projects. They are either installed by default in Fedora Workstation or widely used. Its purpose is to help translators to correctly prioritize their work.

If you want the most important project, that would be the anaconda installer.

Essential Projects

Websites

If you want to translate Fedora websites, you should start with start.fedoraproject.org and getfedora.org.