The default language in SharePoint and OneDrive have impact on eg. spell-check in Office online apps.

To check the default language in SharePoint online install the PowerShell module for SPO (https://www.microsoft.com/en-us/download/details.aspx?id=35588).

Connect to the SPO-service:
Connect-SPOService -Url https://TENENT-admin.sharepoint.com

Ones connected run:
Get-SPOSite -Detailed -Limit All | select Url,LocaleId

You can find a list of LocalID uses in SharePoint here:
https://msdn.microsoft.com/en-us/library/ms912047(v=winembedded.10).aspx