My previous post was lost as it take me a long time to test and write it the same time.
i HATE THAT !!!!!!!!!!
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(short version GRrrr)
Strange caracters and garbage in language.php Fr was the right track !
I have clean it by suppressing garbage and strange caraters (at least around the validation email). After cleaning the cache "template_c" (thanks Sylvie) it was ok.
As i have keep my previous version of language.php FR i put it back to test it and again the validation email was cut.
I have now 2 files of language.php FR
One good, one not.
Someone want to look at it to check the problem ? (of course i'm ok to contribute, but my knowledge is... what it is )