add_cc4_links.py is unreliable and inconsistent

[TimidRobot]

Describe the bug

add_cc4_links.py (current version 564146d as of 2020-01-02) is unreliable and inconsistent. It appends a link for the language specified in the command line argument. This means that the languages are ordered arbitrarily and if a translation branch is created before another translation branch is merged, it won't have the link for that language.

Expected behavior

The script should remove all language links and replace it with the current list (sorted). Ideally, one per line to make diffs easier to read.