Assume following scenario:

• You install an English Windows 10 image and inject a language pack into your deployment
• You block Windows Update and/or Windows Store
• Some built-in apps, which are being serviced through the store (like calculator, camera, etc) remain in the English language

The cause, as it often is with Windows 10, is not straight forward:

UWP apps use language metadata, which you can see / download through the Windows Store for Business. This metadata information is not part of an UWP app provisioned during the initial Windows installation, and therefore is not included in the lp.cab. These metadata bits are actually downloaded from the Windows Store in conjunction with Windows Update (WU triggers the store). This can take up to 48 hours after a language switch.

If you blocked the store via a Computer GPO apps won’t get updated and will remain in their original language. However, if you block the store using User GPO instead, then the Windows system can access the Store and download necessary language bits.

Additionally, you will need to allow Windows Update to contact Microsoft's content delivery gateway tlu.dl.delivery.mp.microsoft.com to update the Windows Store Apps. For more details, read this blog post.

Alternatively, you might be able to get around this issue with offline packages available from Windows Store for Business.