You don't need soft hyphens with your Kobo. What you need to do to get better hyphenation results is to look at the hyphenation dictionary and see the settings in the hyphenation dictionary is too big (like it is with the US hyphenation dictionary) and fix it.
The problem is that
Code:
LEFTHYPHENMIN 5
RIGHTHYPHENMIN 5
(US default setting) is too large and thus, you don't get hyphenation except on really big words. I have mine set both to 2 and it works much better. I'm guessing the Frenchdictionary could be just as misconfigured.