"Post in thread 'x'" is the standard title we use to describe a post in thread and that is passed into the {title} bit of the "Originally posted in:" phrase.
"Posted in" and "post in" may seem a little awkward but I think it makes sense.
If this seems too awkward when translated into French I think it'll just be a case of adjusting your translation.
"Post in thread 'x'" is the common phrase used in more places so I'd recommend adjusting the xfmg_originally_posted_in_x phrase.