How to specify default Ubuntu 16.04 installation language in isolinux/lang?

I previously used this command to select english as the default language for my unattended Ubuntu installation in 14.04 and it worked perfectly:

echo en >> isolinux/lang

For Ubuntu 16.04 this doesn't work anymore. I have tried it multiple times with different permissions. I still see language selection window during unattended install.

Does anyone know how it should be done in 16.04?

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