In T253984#9761334, @Tacsipacsi wrote:

Two things can be done:

• Warn the user (warning)
• Warn the user and don’t allow submitting the form (error)

And @abi_ listed two cases:

• […]
• Group with priority languages.
  • Group with enforced priority languages. Probably it shouldn’t be allowed to send notifications to non-priority languages (error). This means that when a group with enforced priority languages is selected:
    • All languages should be changed to say All priority languages
    • All languages except should be changed to say All priority languages except and only allow selecting priority languages
    • Only selected languages should only allow selecting priority languages
    • The no-JS experience should at least filter out non-priority languages and reject a request if no language is left, but preferably display a confirmation if any languages were filtered out (but some remain).
    • Consequently, if a group has zero priority languages (T359975), no notifications can be sent about it.
  • Group with non-enforced priority languages. I think it’s enough to warn the user if they are about to send notifications to non-priority languages (including when they use All languages or All languages except). An extra convenience could be an option in the warning to switch to Only selected languages and fill in the field with all priority languages with one click.

In T253984#9773471, @abi_ wrote:

I think that summarizes my thoughts well.