macOS can use a different language for individual apps. The setting is easy to miss: it lives in System Settings, under General, then Language & Region, in a section called Applications.
It helps anyone who switches between languages or regional spellings during the day. The usual case is British and US English. Spellcheck can’t flag a correct word that belongs to the other version of the language, and a per-app language is a partial fix for that.
It also helps someone learning a new language. Switching one everyday app to that language adds daily practice without changing the whole system.
To set it up:
- Open System Settings, then General, then Language & Region.
- Find the Applications section.
- Add an app, then select a language for it.
Browsers are the useful target, because much writing happens in one. Two browsers can cover two spellings. With British English as the system language, Safari uses it, and Chrome can be set to US English. Writing in one browser or the other then picks the spelling.
One limit applies. Only apps that support macOS’s localization APIs can be set, so some apps never appear in the list.
Open that pane now and pick a language for the app used most for writing.