I’m having a problem that spellcheck keeps trying to use Euro spelling, z’s instead of ‘s’. I.E. Realise instead of realize,
How do I turn this off? This happens in both Preferences/Language/ English or System Default.
Auto - complete doesn’t work.
Mac 10.13.6.
What do you have set as your System Default? Mine is set to “English (UK)” but you can set it to other varieties of English like “English (US)” via System Preferences > Language and Region > Preferred Language. If you don’t see the variety you want on the left hand pane, click the ‘+’ button and choose it from the list.
Also, if you go to Edit > Spelling and Grammar > Show Spelling and Grammar, at the bottom of the dialog is a dropdown menu from which you can choose the variety if you want it different from the system default.
I don’t use auto-complete, so I can’t help you with that. Perhaps someone else will chip in.
HTH 
Mark