Biiiiiig thanks to you j-b-m for your priceless help, I changed my locale to EN-Gb, and now everything is fine.

Ok, so now what is the result of:

locale LC_NUMERIC

For me, it is:
.
,
3;3
46
44
UTF-8

You can see that the first character is a dot (.) which is used as decimal separator. If your locale uses a comma (,), then it might be the problem. If it is so, can you try to change your locale, see this post to get a dot as separator:

http://forum.ubuntu-fr.org/viewtopic.php?id=81122 (it's in french but I guess you understand)

Then check if it solves the issue in MLT / Kdenlive for you (if it does, we will have to do a real fix in MLT).

Oh, ok you were faster than me on that one.

Then we need to fix that for real in MLT, nice to have a confirmation for that locale bug.

regards

look at the post #16

if the results are different then change your loclization to something like English (EN).

( by the way you pasted a wrong command line here )