Change writer default spellcheck to English (USA)

Tried forum solutions. Tried via templates. None work.
How do I set the Default language for Check Spelling. I have done Tools / Language Settings / Languages, changed Default Languages for Documents to English (USA), checked Ignore system input language, [OK] - but it only lasts while I’m editing that document. Subsequent Writer session have the default revert to English (UK). How do I make the Default Languages for Documents permanently English (USA).
LO 5.4.7.2 (28-Jul-2018) Do I need to upgrade?

Missing: OS name and save format of your documents.

Changing Tools>Options, Language Settings>Languages should be enough unless you have a modified default template (which is likely if you end up with en_UK). What does Default Paragraph Style (I don’t remember its name under 5.x) Font tab say about language?

Note that 5.4.7.2 is a very old release which has reached its end of life.

Yes, you should:
Download LibreOffice

Thank you. I never knew about Styles, and LO says “LibreOffice is a styles-based program.” so that’s probably my problem I thought one set defaults via Templates. Looking at Styles right now, I’m bewildered. It’s too complicated for me to deal with the 97 elements under Styles and Formatting, with multi-tab modal boxes for modification (and there are elements I’ve can’t figure what they mean, e.g. Numbering 2 End). BTW, I don’t see language in the Default Paragraph Style display for All Styles, but maybe that’s because I have an old version. But thank you for suggesting the likely problem that I’m dealing with. I’ll probably ditch the templates and set everything manually from now on.

And you’ll head into even worse problems. Direct formatting is the ideal way to reach formatting nightmare.

Don’t be bewildered by the wealth of built-in styles. They nicely group into families. Select hierarchical view from the bottom drop-down menu in the style sidepane (I don’t remember the wording for this menu in 5.x).

Anyway, start easy: read the Writer Guide for an introduction. Once you get familiar with styles, writing a document is a treat.

Right-click on Default Paragraph Style (perhaps, it’s simply Default Style in your release) and Modify. Go to the Font tab and look for a Language drop-down menu.

But notice, upgrades usually keep your profile. So what you set as defaults in your old version may also “pollute” your next (not entirely new) installation.