Hi,
Thank you for posing in Q&A forum.
For adding language pack in WCD, please refer the link below:
An easy way to add Langauge Packs to Windows 10 1511
For configure default language pack, please check "Configure international settings by using DISM" part in the following link:
International Settings
If you have any other confuse, please reply directly. If my reply is helpful, please take it as answer.
Note: This is a third-party link and we do not have any guarantees on this website. And Microsoft does not make any guarantees about the content.
Bests,