Purpose
=======
When creating an opportunity, set the language of the Lead/Opportunity
to the partner's language if it is set instead of leaving it blank.
Also update tests to correctly test lang propagation.
Update event_crm so that lang of lead from registration is directly set
to False when there is no partner. Indeed we have no clue which lang we
should set and we can skip the field computation that otherwise triggers
some additional queries.
Task-2709436
Part-of: odoo/odoo#81028