TOPIC:

I thought I've understood, but no.

I am on a multilingual site. When the menu ID of eventslist is in the settings, I get an error 404 in italian and in french. In german it's ok.

When there's no menu ID in the settings, I've no such problems: in each language I get the right page.

What for is the default menu Item ID?

The default menu item id is for routing detail views of events.
For example you have the following menue:
Main Page (id:1) - Events Simplelist (id:2) - ...

On the main page you have a basic module showing events. If you set up the default menu item id to 2 and now click in the basic modul on the event "Testevent" you will be routed to <url>/eventssimplelist/event/<eventid>-testevent. So it is more SEO compatible then without the configured default menu item id because the event from Main Page -> Basic Module -> Testevent has the same url as Events Simplelist -> Testevent.

If you do not have set up the default menu item the url from the basic module to the event would look like <url>/component/jem/event/<eventid>-testevent. So the same event has two different urls:
Main Page -> Basic Module: <url>/component/jem/event/<eventid>-testevent and
<url>/eventssimplelist/event/<eventid>-testevent.

Is it understandable?