I think I found some way to change the default settings, whatever they are, I mean document language but also default character style and so on.
It looks like the templates are stored in a data/appdata_something/richdocuments/empty_templates/ as of LOo templates, that is:
- document.ott
- presentation.otp
- spreadsheet.ots
So you need to locate them, download them and modify them with your settings, save them as templates (ott, otp, ots) and upload them back to the same directory caring to let them to the directory owner (www-data:www-data for instance).