I experience the same as you do. I'm quite sure it's because we're not as used to speaking in the second language (French in your case) as our everyday-tongue, so all the little tricks we use to avoid a block (switching out the word, manipulating the intonation or using "hmm", "umm" etc) we don't really know how to use in the foreign language. Also, you' have no "choice" in what you're going to say in French in regards to words, you first have to figure out actually which words are correct to use, their conjugation and sentence placement and only THEN can you start to actually speak them. - What if you're gonna block on a word? its gonna be hard to evaluate 5 synonyms in a splitsecond in your brain like you would in English! So yeah, it sucks! I'm half French, and even though I actually know and speak the language quite well, I rarely use it in real life and I sound much, much worse at it because my stutter holds me back :(