This is not a command the end user would want to run. Ever. This documentation is meant for people who are studying the Porcelain-ish scripts and/or are writing new ones.
The git sh-i18n scriptlet is designed to be sourced (using .) by Gits porcelain programs implemented in shell script. It provides wrappers for the GNU gettext and eval_gettext functions accessible through the gettext.sh script, and provides pass-through fallbacks on systems without GNU gettext.