You can change the device language and unit system from the settings under General »Device settings.
You can change these settings at any time. Suunto EON Core refreshes immediately to reflect the changes.
Under the unit settings, you have the option of selecting metric or Imperial as a global setting, meaning the affect all measurements. Alternative, you can choose Advanced which allows you to set the unit system for specific measurements. For example, you could use metric for depth, and Imperial for tank pressure.