[HirnHorn]

Or my favourite easy version without needing to download anything:

Open Terminal and override your primary language for OmniFocus to be english:

defaults write com.omnigroup.OmniFocus AppleLanguages '("en-US")'

Works with most applications in ~Library/Preferences. Just look for the entry com.companyname.softwarename.plist and leave out the .plist at the end