One of the most common error people do when configuring Office 365 is doing so with the wrong default language! And trust me this is so common that we are doing language migrations almost weekly.
It might sound like that’s just something that you change like a setting but the default language is actually set when you configure the tenant for the first time and is something that you can not change after the initial provisioning is made.
Or actually it is possible to change it but that requires a reset of your default sites in SharePoint and often a migration of OneDrive for Business data etc.
Running the following PowerShell Command will list all your sites and the language they are created in.
Get-SPOSite -Detailed | select Url,LocaleId
If you have a LocalId other than 1033 on some of the default sites you are in trouble and our recommendation is to contact us to resolve this.