kanaye wrote:

They have speculated that the phone has been bricked during the update because it is not compatible in EU (yet it is marked in your product sheet "EU")

When electronics here have EU in the product title it usually only means that the provided electric plug is EU compatible.
Example from the FT specifications for SKU 9701069:
PACKAGE CONTENTS ... 1*EU Plug Power Adapter ...

What the Xiaomi France representative probably meant was that the Redmi K20 you bought here isn't intended for use in the EU as their EU version of the K20 is named Mi 9(T).

Not that this helps you much in your current situation. :/
As SOLOMONgrundy wrote, the decent thing to do would be to refund the amount you paid for this phone (once it has been returned to them).