It doesn't use English when other language is available as a secondary language.
Reproducing:
1. Open "Language & Region" in macos settings
2. In "Preferred languages" box, set English as a primary language.
3. Add another language, that git is translated to, as a secondary
language, for example, French or German.
4. Run any git command - git will use the secondary language, instead
of English.
When the secondary language is removed, then git starts using English again.
I have git 2.19.0, installed from brew, and my OS is macOS 10.13.6 .