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Unfortunately, there is currently no supported method to change the primary language of an existing SharePoint Online root site after it has been created.
Since you have already contacted Microsoft Support and they confirmed that the tenant's default/root language cannot be changed after tenant creation, there is no additional SharePoint setting that I can recommend overriding that limitation.
However, there are some supported alternatives depending on what you are trying to achieve:
- Create a new SharePoint site with English as its primary language. The primary language is selected when the site is created.
- Replace the current root site with the new English site. SharePoint supports replacing the tenant's root site with an eligible site by using the supported root site swap process. This changes which site is served at the root URL, but it does not change the tenant's original provisioning language. You can read more at Modernize your root site - SharePoint in Microsoft 365 | Microsoft Learn. Microsoft has changed the SharePoint Admin Center experience over time, and many tenants no longer expose a Replace site button. The supported root site swap operation can still be performed using SharePoint Online PowerShell via Invoke-SPOSiteSwap.
- Configure SharePoint regional settings where the issue is related to date, number, or currency formatting.
- Configure users' Microsoft 365 language and regional preferences where the behavior is user-specific.
- Use SharePoint multilingual features where multilingual content or navigation is required.
For example, you could create a new Communication site with English as its primary language and then perform a supported root site swap to make it the root site. However, it is important to distinguish this from changing the tenant's language. A root site replacement does not convert the tenant's original default language or automatically change all Microsoft 365 localization behavior.
Therefore, if the requirement is specifically to change the tenant's original/default language configuration, Microsoft currently does not provide a supported post-creation setting for doing so. If the requirement is instead to provide an English-language SharePoint root site experience, replacing the root site with a newly created English-language site may help, although it may not resolve all of the localization scenarios you've described.
This limitation is a known customer concern, and a feedback request has already been submitted through the Microsoft Feedback Portal. If you are also affected by this limitation, please consider voting for the existing feedback item to help increase its visibility with the product team: Allow changing the base (default) language of an existing SharePoint Online site.