My prefered language in SQL Developer is English.

If the language is not English here are the steps to change this:

Add the line following line to the file sqldeveloper.conf in the directory sqldeveloper\bin\. On a Mac it is in the SQLDeveloper.app in the folder Contents\Resources\sqldeveloper\sqldeveloper\bin\

AddVMOption  -Duser.language=en

Restart SQL Developer.

Versions: 4.0.0 to 21.4.3