We have the (undocumented?) $CFG->skiplangupgrade = true config option.
Unless I'm wrong, it was created to avoid the language packs to be updated as part of the upgrade process.
Instead, we are using it to:
1) Prevent the use of the import languages UI (admin/tool/langimport).
2) Prevent the execution of the update_langpacks_task task.
3) Prevent the update of langpacks in upgrade (lib/upgradelib.php)
And only the 3rd use above seems to be the correct one.
So this is about:
A) Document the configuration setting where appropriate.
B) Review the uses above and decide if all them are valid or things should be controlled in another way (new setting, unconditionally, whatever...)