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\
Restart SQL Developer.
Versions: 4.0.0 to 18.2