Ok, I understand the issue: When the RDF do not include lang for own data, you will need to specify what is the "default" lang of the RDF and it could match not with the Dédalo installation default lang.
The RDF asume that his own data is in English but your Dédalo installation asume that default data is in Spanish.
...and you know, that you can change the lang before import, but, sometimes you or your team forget do this change...
Ok, let me think about this, because the RDF import tool is not only for Nomisma RDFs and it's not only for English world. Maybe, a solution could be add a Lang control inside the import tool. So, when you click the import you will see the default lang and choose if you want import in English, Spanish, Arab... etc.
I will back with a "proposal".
For the "Key" field (numisdata81) inside Type?, will need more quorum to do this change... @Manuel Gozalbes, @Pere Pau, @Markus, What do you think about that?