As a standard SQL Developer uses the system OS language in order to determine it’s own language setting. In my case this is normally German. Unfortunately the German translations rather confuse than help me to understand the UI. Therefore, I tend to switch the UI language to English each time.
This can be done quite easily by adding a new line to the $SQLDEVELOPER_INSTALL_DIR\ide\bin\ide.conf file:
AddVMOption -Duser.language=en
Thank you. Simple and Immediate.