When the calendar plugin is enabled and you visit the /preferences/profile page through the interface, the save changes button disappears and I see this in the console:
If I go to the profile url directly I see this in the console:
AI-generated summary
The issue involves the save button disappearing on the profile page when the Calendar plugin is enabled, with console errors indicating a problem with the region-input component. davidkingham reports the issue persists even in safe mode and with only the Calendar plugin enabled, but it resolves when the multilingual plugin is disabled. pmusaraj suggests checking for conflicts and asks for console output of available_locales, which returns a valid array. davidkingham notes the issue mysteriously resolved, and davidkingham confirms it was caused by a conflict with the multilingual plugin.
The issue occurs here:
Unfortunately, I don’t know where the issue comes from (I don’t encounter it), but maybe it will give a clue to someone else with better knowledge 
I can’t reproduce this on meta. Can you try either with safe mode or by disabling other themes or plugins. I suspect something else might be interfering.
I was afraid of that. When I use safe mode disabling unofficial it still does this, but when I disable all plugins the issue goes away. Then I tried disabling all other official plugins except calendar in /admin/plugins and it still happens. I also cannot duplicate this on my sandbox so it seems to be something specific with my site and the calendar plugin. I also get this error when enabling calendar Unable to configure link to 'Calendar'. Ensure ad-blockers are disabled and try reloading the page. and ublock is disabled.
On the broken site, can you run this in the browser console and share the output here:
JSON.parse(Discourse.__container__.lookup('site-settings:main').available_locales)
I ran that on this page on an NPN page (as an anonymous user), and it returns an array of 25 languages (with names and values), which seems fine.
FYI this was a conflict with the multilingual plugin. I’ve disabled that and all is well again.