Hi @Alex Carlock ,
Great to know your issue has been resolved and thanks for sharing the update here.
By the way, since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others.". and according to the scenario introduced here: Answering your own questions on Microsoft Q&A, I would make a brief summary of this thread:
[Why can't the SharePoint Online tenant default language be changed?]
Issue Symptom:
Our tenant was originally created in Dutch and we want to change it to English so all SharePoint site creation (via SharePoint, Teams, O365 Groups, OneDrive, etc.) default to English. Changing this is not currently possible.
Does anyone know why Microsoft has not enabled a setting to make this change? Is there some back-end design of SharePoint/M365 Tenant that makes it impossible for them to implement this? If so, does anyone know what it is?
Current status:
We finally got to a helpful engineer at Microsoft and they deleted our root site and re-created it with our new preferred language. That means all future sites default to the new default language.
They told me that during the root site provisioning, many site/page templates are coded with the chosen language. They couldn't tell me why SharePoint is designed like that, but at least now I know that when adding new sites, various components are copies of the templates stored in the root site, rather than being dynamically built/configured during site creation.
My suggestion to anyone else is to push your engineer to delete the root site and re-create it. Keep in mind that any content and setting in the site will also be deleted, so you need to prepare for that. In our case, we had very little there and virtually all our content is in other site collections.
You could click the "Accept Answer" button for this summary to close this thread, and this can make it easier for other community members to see the useful information when reading this thread. Thanks for your understanding!