After obtaining a new notebook from my company, I reinstalled my developer tools, including Oracle SQL Developer.
My Notebook is configured in French, and the outcome was that SQL Developer was also configured in french, which is not what I wanted - decrypting Oracle server errors in french is not very helplful.
There is no global preference or setting where the language can be set.
I found a solution in this post in Stackoverflow:
add the following option to to the file sqldeveloper.conf file located under %sqldeveloper-home\sqldeveloper\bin :
AddVMOption -Duser.language=en
