There is an option to export the SharePoint Online default language. (https://contoso.sharepoint.com/_layouts/15/exporttranslations.aspx)
So I have made modifications the default language but it can't be imported, only the additional languages can be imported. (https://contoso.sharepoint.com/_layouts/15/importtranslations.aspx)
And also I would like to know how can you make bulk changes to the SharePoint Online languages?
Because changing language between English and an another language is not instant in SP, and it really annoys me.
My plan is to turn it off but currently I did not find any option for that. :(