You're the first one to mention this, so it's never really thought about. The interface is mostly written in en_GB. The languages you can choose from in the preferences dialog are en and en_GB, but this is wrong. There is no en translation, it's just the default. When selecting en, no translation is applied, so it's basically already en_GB. If you can still follow...
There's 2 options at the moment I think:
1. Change the strings to en_US by default. Keeping an en_GB translation file.
2. Keep the current en_GB as default, remove the en_GB translation file (it shouldn't be doing anything anyway) and add a en_US translation file.
Can we perhaps create a list of strings in the application that are different between en_GB and en_US? I know "Colour" is the obvious one, but are there more?