New comment by oreo639 on void-packages repository

https://github.com/void-linux/void-packages/pull/39264#issuecomment-1369171391

Comment:
Because GDM sets LANG and then gnome-session launches bash as a login shell so that `/etc/profile` can be applied which overwrites the previous LANG preference.

This PR adds a check for if LANG is already defined (i.e. by the user/DE/etc) before applying the system-wide preference.