Installing language packs during Operation System Deployment (OSD) for Windows 10.
Unfortunately, with a Task Sequence you can only make one step to add a language to the base image, otherwise an error message will appear on the second step…
It is possible to add multiple language packages in one package, I have done this for Dutch and German.
You can download LP (Language Pack) ISOs for Windows 10 Enterprise 1511 / 1607 / 1703 / 1709 from the website below:
https://blogs.technet.com/mniehaus/2017/04/26/finding-windows-10-language-packs/
The language packs nowadays do not have a generic file naming called “lp.cab”, but “Microsoft-Windows-Client-Language-Pack_x64_nl-nl.cab”.
In the answer file (unattended.xml) you can indicate which language pack should be set as default.