On MAC High Sierra (10.13.6)
cd /Users/vkrishna/.sqldeveloper/18.2.0
nano product.conf
on the last line add
AddVMOption -Duser.language=en
Save the file and restart.
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If you are using standalone Oracle Data Modeller
find ~/ -name "datamodeler.conf"
and edit this file
cd /Users/vkrishna//Desktop/OracleDataModeler-18.2.0.179.0756.app/Contents/Resources/datamodeler/datamodeler/bin/
Add somewhere in the last
AddVMOption -Duser.language=en
save and restart, done!