See: https://forum.joomla.org/viewtopic.php?f=711&t=970653#p3563278

Steps to reproduce the issue

Install a multilingual site with 3 languages minimum, one of them being the Indonesian language.
(This can be done during a clean install of staging)
If not yet done, make sure the Indonesian content language exists.
If not yet done, create a specific main menu for Indonesian, containing a Home page and any other menu item tagged to Indonesian.
After saved, the Associations tab will display. Click on the Tab.

Expected result

There should not be any choice for Indonesian as the item is tagged to it.

Actual result

The Indonesian Content Language is proposed as a possible association.
For menu items but also for any type of items: categories, articles, etc.

Additional comments

This looks related to the Indonesian language tag : id-ID, specifically the id language iso code.
To test, I created a modified Indonesian pack with tag mid-ID and it works fine.