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I was able to find a quick-fix for this,
its rather dirty but im not going to proper debug this now.

go into your inc/ directory,
then open sql.inc
go to line 1084 (or search of the the SQL_UpdateUser function)
find the following part
(should be underneith the function SQL_UpdateUSer statment):
"Global $sess,$sess_pwd,$_MPM,$db; $_MPM['Serial'][]"

now underneith this line we add the following:
if($set == 2 && $tset2 == 19){ $set = '';$pid = '';}

Personally this is a nasty fix and will block you from ever setting your language as french again, but it fixes this particular issue until the devs of this projects (if ever) will fix this.