This is not a command the end user would want to run. Ever. This documentation is meant for people who are studying the plumbing scripts and/or are writing new ones.
git sh-i18n--envsubst is Gits stripped-down copy of the GNU envsubst(1) program that comes with the GNU gettext package. Its used internally by git-sh-i18n(1) to interpolate the variables passed to the eval_gettext function.
No promises are made about the interface, or that this program wont disappear without warning in the next version of Git. Dont use it.