Oracle SQL Developer does not allow you to change the default language on the GUI. No idea why, but that is a given.
To solve this issue, go to your installation folder, and then to the \ide\bin folder, e.g.: C:\oracle\Product\sqldeveloper\ide\bin
In this folder, make sure to edit the file file ide.conf with a good editor like Notepad++ and add the following line in the end:
AddVMOption -Duser.language=en
The next time you start SQL Developer, your menus should be in English!
