...or even in a slice without the loop. So:

for my $eng (keys %hash){
   $hash{$hash_tr_fr{$eng}}=$hash{$eng};
   delete $hash{$eng};
}
[download]

becomes:

my @temp = keys %hash;
@hash{@hash_tr_fr{@temp}} = delete @hash{@temp};
[download]

Update: corrected missing @, thanks SuicideJunkie