"header" Message Stack Error in welcome email tool

Ok... Another little bug on my side!

The cart seems to work well. I'm workin on a brand new installation, of ZC 1.3.8.

I'm workin on a french translation. To do so, I started by rename the "english" directory and the "english.php" file to "french" directory and "french.php". I didn't change nothing for now... I only use the englsih files as if they were fench. I did the same for the admin language files...

Everything works well to choose the new language.

BUT... In the admin section, when I go to "tools>email welcome" a pink stripe appear at the top of the page, it is a tr class="messageStackError". The only message I can read there is "header"...

If I switch to english to see the email welcome tool, the stripe disapear...

So, my question is : what ZenCart is trying to tell me with this "header" Stack Error ???

... From what I can tell, I did have no problem to send e-mail, I tried a lot of test subscription and I always reveived the welcome email. I can live with this pink stripe since I don't really need this but I suspect a problem somewhere deeper...

Thanks!

S.