British English instead of American default?

Welcome to the forums.

In Scrivener for Mac:

  1. spelling and grammar should respect the setting in:  → System Preferences → Keyboard → Text → Spelling → declared language

  2. the user interface at Scrivener → Preferences → General → Language → Interface Language → declared language allows users to change the language Scrivener uses for menu items. It does not change the language used for spelling and grammar. The English used in the interface is mainly US English, with spellings such as ‘color’. Your actual writing space should be in whatever language is set in System Preferences

I don’t know how things work in Windows. Possibly this: