After the migration it was discovered that the source and target language is different.
The source language of a SharePoint Site was not maintained when it was migrated with On Demand Migration.
First of all, it is important to check what kind of SharePoint Site was affected, since the behaviour can be different based on if it is a standalone SharePoint Site, or a supporting SharePoint Site or the Root Site.
In terms of Standalone SharePoint Sites, the Microsoft SharePoint API allows for the language to be specified when creating a site. This is reflected in the MS SharePoint Admin Center UI where site language can be selected when a site is manually created.
On Demand Migration SharePoint is designed to maintain the source language of the source site during migration. Please note, the default language from the source site is maintained on the target for sites created during a migration. The default language for sites that have been pre-provisioned on the target will not be updated during a migration if it is different from the source default language.
In terms of SharePoint Sites which are associated with a Team, Channel or M365 Group, when a Team or M365 Group is provisioned, the language of the supporting SharePoint sites is set to the default target tenant language by Microsoft. There is no way to control the language of the supporting SharePoint site, so they will always be created with the default tenant language.
This behavior is expected and stems from how Microsoft provisions multilingual SharePoint sites. Quest ODM respects these structures and provides mapping logic to align libraries during migration. While it may seem like a limitation, it is actually a safeguard to preserve content integrity across languages.
In terms of the Root Site ('/'), we have no control over, as this site is automatically provisioned by Microsoft when the M365 tenant is created. As it is pre-provisioned by Microsoft we can only migrate into it and as mentioned above, the language of a site cannot be changed via a migration task, if it already exists.
If the language of the root site is not the desired one, a new Site Collection can be manually created on the target with the desired language and then it replace the root site. Note that existing root site is moved to a managed site collection in the replace procedure.
