There are quite a lot of tools available to do this in a generic and reliable way from CPAN, but from what you describe, I think this comes close:

my $tgt = "fr_fr";

my %day = (
    en_gb => [qw( sunday monday tuesday wednesday thursday friday satu
+rday )],
    fr_fr => [qw( dimanche lundi mardi mercredi jeudi vendredi samedi 
+)],
    it_it => [qw( day0 day1 day2 day3 day4 day5 day6 )],
    );
my %tr = map { lc $day{en_gb}[$_] => $day{$tgt}[$_] } 0..6;
my $tr = do { local $" = "|"; qr{\b (@{$day{en_gb}}) \b}i; };
open my $fh, "<:encoding(utf-8)", "db-dates/$src" or die "$src: $!";
while (<$fh>) {
    s/$tr/$tr{lc $1}/ge;
    print;
    }
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