Language support keeps disappearing from time to time.
I suspect this happens after “bigger” updates to Cinnamon etc.
I usually notice when dates in Thunderbird and other details start to look whacky.
This is how mintlocale looks like in the “languages once again lost” state; notice the missing icons:

In those cases I have to manually reinstall the locale that went missing;
i.e. I have to open mintlocale, click on the “Install/Remove” button, press “Add…” in the install/remove window and then browse the list for the locale to add, confirm this action and wait.