Documentation - Language packs
Integrate language packs - This page can be used to integrate language packs into the image so that you can choose them under the Unattended - Regional - GUI language and have installed and set automatically.
First start by checking the Enable checkbox to enable the page. Then use Insert to queue language packs on the integration list. If you do not have LP.CAB then you can run your downloaded LP.EXE and when it is running it will have LP.CAB in the same directory, just copy that aside and integrate on this page.
Already integrated hotfixes - This list shows the already integrated languages.