If you use Windows 10 language packs, the sources in the MultiLang-iso-files do not contain language updates for the Inbox Apps.
In general, they should be updated by a scheduled task (Microsoft -> Windows -> Windows Update -> Automatic App Update), but this task requires the Store app to be accessible and the machine to be allowed to connect to Microsoft Store URLs. So, if you have to block the access to the Store app for users or running machines in critical areas w/o any internet access you have to stay with English for the Inbox Apps and the Inbox Apps will update only with the next feature update.
Even if you have access to the Microsoft Store, the update process is not always working correctly. If you have multiple language packs installed, the task tend to pick the wrong language, e.g. Windows 10 English International + French + German (Swiss configuration) with fr-CH as Windows display language, updates the Inbox apps to German.
In Windows 10 1804, we have Local Experience Packs that will also update the Inbox App language. Tests on the insider builds showed just some minor issues in the Start Menu, some apps like Calculator or Camera stay in English until a user opens the apps.
However, the Local Experience Packs are Online only, so we cannot import them into Configuration Manager to deploy them to critical no-internet clients. And we still don’t have a solution for updating the Inbox Apps on these clients.

As always, we need the Configuration Manager team to the rescue. Pls. support uservoice:  https://configurationmanager.uservoice.com/forums/300492-ideas/suggestions/33999229-manage-windows-10-inbox-app-updates-and-language-v