Remind people to fill in full descriptions in both languages in the event backend

One-sentence description

Add a reminder to the event description fields that people need to fill in the descriptions in both languages.

Motivation

Some Dutch-only events have an English description "This event is in Dutch", without the actual Dutch description. This forces people to change their language to Dutch if they want to see the description.

Desired functionality

A small notice at the description input fields, with something along the lines of "Don't forget to include a full description in both languages!".

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