If you don’t want to change the languages in Tools
>Options
, Languages & Locales
>General
, I am afraid you can’t unless your documents are based on a custom template.
The default “factory template” is not mentioned if the document metadata (same omission when you double-click on a template document to create a new one; template association is kept only with File
>Open
>Templates
). Therefore, changing paragraph style Default Paragraph Style in the template will have no effect on existing documents.
If our reluctance to change “system language” is a fear to alter the UI, note that you can configure separately the UI (e.g. keep en_US), the locale (date, currency, …) and document language (e.g. switch to en_UK).
I have not experimented to see the effect on existing documents. I’d bet it won’t change the language. But take the opportunity of an edit to change the language in Default Paragraph Style
Font
tab to switch to a new one. Default Paragraph Style is a “technical” style whose attributes are shared by all other styles. Thus any change cascades down (unless it is overridden in a particular style).