An "Archicad Language Pack" typically refers to two distinct things:

The Complete Guide to Archicad Language Packs and Localization

: Specific library packages ( .libpack ) that contain translated object names and parameters for the standard Archicad library. Why You Can't Simply "Change" Languages

Managing languages in Archicad is often misunderstood because, unlike many modern applications, the software does not use a simple toggle to switch its entire user interface. Instead, Graphisoft employs a system of localized versions and specific library packs to support its global user base. What is an Archicad Language Pack?

Unable to change the language in Archicad? - Graphisoft Support Site

If you need Archicad in a different language than your current installation, follow these steps: