Hello @Hanii
For flatpak installation document templates are located in ~/.local/share/flatpak/app/org.onlyoffice.desktopeditors/x86_64/stable/active/files/bin/opt/onlyoffice/desktopeditors/converter/empty. In this folder you will find other folders named after the interface settings, e.g. es-ES for Spanish, fr-FR for French and so on.
You can copy all three files new.docx, new.xlsx and new.pptx from folder en-GB (English - Great Britain) to folder en-US (English - United States) so Desktop Editors will be using these documents as templates for creating new documents.
In general, you can edit these templates in en-US folder to your liking.
Another way, you can create a document with all desired settings and place it into /home/Templates directory. After that you will be able to create documents based on this template via contextual menu > New document and selecting needed template.