I discovered a hidden feature in System Settings that lets you set different default languages for individual Mac apps. Located at the bottom of System Preferences > General > Language & Region under an Applications section, this feature allows you to assign specific languages to supported applications. Apple's US centrism is evident here, labeling US English as "English – English" while British English appears as "English (UK)".

This becomes particularly useful for bilingual users or those working with different English variants, like British versus US English spelling. I use this daily—Safari runs in British English while Chrome is set to US English, helping spellcheck properly identify correct terminology for each variant. However, there's an important limitation: only apps that support macOS's localization/language APIs will appear in the list, so not all applications are compatible with this feature.