Not possible to determine valid language for ISO code &

What causes this issue?

The system cannot process the IDoc because no language has been
maintained for ISO code &V1&.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Get your supplier to send you a new IDoc with a valid ISO code and a
valid language.

Error message extract from SAP system. Copyright SAP SE.