hey stef,
first of all I can't rate the french translations as my last french lesson in school was years ago but I'm sure they're doing the best they can.
but now something about your probs and pics:
for the streams with the english "created a blog post" that's for example not a fault of the translators, the blog app you have installed has this string hardcoded in the theme files so the translators CAN'T translate them. to get this fixed download the latest blog app where this problem is solved and additionally adjust the language files (there were 2 new strings you would have to translate manually right now as the translation files at transifex weren't updated yet)
for the time format in your pic this is also something that's coming directly from the theme where translators can't do anything about.
but the biggest issue with the "poor" as you said sentences a small explanation might be helpful:
when you see a sentence like "you like this" and the french (also other language translations including our german one's) might be wrong. that's not because the french translator wouldn't know how to do it right - instead this sentence doesn't exist for a translation. instead in the about 70(!!!!!) language files a translator can translate the word "you" as a single word, then he can translate the word "like" and again the word "this" - all of these translations itself will most probably be right but later these words are combined in the system and THEN you get the bad results you see now.
let me tell you that this is frustrating not only for users but also for translators who put A LOT of time and effort into this but it's nothing that can be controlled right now. however, the good news is that stackideas team already knows about these language problems with combined strings and with cached (in the database stored) entries that are hurting a good user experience in terms of multi-language and will have this fixed in easysocial 1.2. there these sorts of strings will finally be available for translators in a single line so that they can be translated in a meaningful way and in addition these messy looking notifications where some are stored in english and others in the native language will be gone.
hope these explanations give you some insight about what's going on behind the scenes and you can look forward to easysocial 1.2 which will be out hopefully soon
and not to forget: merry christmas of course