Language packs are translations that 3rd party authors have submitted for UBB.threads. They contain a form of all of the language strings in the UBB.threads release in the target language.
These downloads are not officially supported by UBBSystems or anyone other than the 3rd party which posted them.
For instructions on how to create a translation, please see the UBBWiki
article "How to Translate UBB.threads