A German (or french etc...) system must be used to reproduce.
It is likely this is not limited to MacOS, but I do not have a non-english Linux system available

An out of the box install of wildfly/EAP:

Without configuration, the log file is in German as expected.

Using these CLI comands:

:read-operation-description(name=stop-servers,locale=de_DE) -> german
:read-operation-description(name=stop-servers,locale=en_US) -> german
:read-operation-description(name=stop-servers,locale=fr_FR) -> french

So we cannot get the CLI to produce english output

when configuring JAVA_OPTS in domain.conf with:

JAVA_OPTS="$JAVA_OPTS -Duser.language=en -Duser.country=DE -Duser.encoding=utf-8

The log is now in English -> works as expected; and:
:read-operation-description(name=stop-servers,locale=de_DE) -> german
:read-operation-description(name=stop-servers,locale=en_US) -> english

So it seems we have a bug where the locale set to start the domain takes precedence over the locale set in the CLI command (but only when English is asked)

I presume this is because English is the default locale.