Show a warning when default language is switched from English

Problem/Motivation

Recently, on a new site, I've switched the default language from English to a different language. I've exported the config and reinstalled the site. I've discovered that English language has vanished. Lately I've discovered that I need to add keep_english: true in my custom profile. Took me 1 hour to find this.

We need somehow to inform the site builder about this before they lose their mind.

Steps to reproduce

See above.

Proposed resolution

If the profile doesn't have keep_english on, show a warning after the default language is switched from English that instructs the site builder what they need to do.

Nice to have: An intermediary screen/form where the user is presented with the warning and is able to cancel until the issue is fixed.

Further reflection: Maybe keep_english is not a profile thing but a site config?

Remaining tasks

Clarify the warning message.

User interface changes

To be clarified.

API changes

None.

Data model changes

None.

Release notes snippet

To be clarified.