There seems to have been a fix introduced since this thread was last written. Now (Feb 2015) you can add languages to your 'available languages' in the settings menus of Swiftkey. If you add the languages in which you type, it downloads word lists for those so that it will recognise valid typing In those languages and not try to 'correct' them into nonsense phrases in English. To get to this setting, touch the SwiftKey app icon, then select Languages from the settings page that appears. You will then see a list of Downloaded Languages and another of Available Languages. Touch a language in the second list and it will - after getting your confirmation - download and activate that language for SwiftKey use.
This solved the problem for me on two Android devices I was using for both English and French (previously it was auto-correcting all French typing into English rubbish)