You might not want the case insensitivity or to allow spaces between the language token and the comma, but I added them for completeness.

#! perl -slw
use strict;
my %validLanguages = (
       "de" => "german",
       "en" => "english",
       "es" => "spanish",
       "fr" => "french",
       "it" => "italian",
       "ja" => "japanise",
       "ko" => "korean",
        "ru" => "russian",
       "sv" => "WHATS THIS",
        "zh" => "WHATS THIS",
        "zh_TW" => "WHATS THIS"
);

my $re_langs = join'|', keys %validLanguages;
$re_langs = qr[\s*(?:$re_langs)\s*(?:,|$)]io;

sub isOnlyLangs{
    my ($string) = @_;
    $string =~ s[$re_langs][]g;
    $string =~ m[^\s*$];
}

sub isOnlyLangs_{ (my $s = $_[0]) =~ s[$re_langs][]g; !$s; }

print isOnlyLangs($_) ? 'Passed : ' : 'Failed : ', "'$_'"
    for
    'de, en, fr, ja',
    ' de  , en  , fr  , ja ,  ',
    'monkish fr, en',
    'monkish, fr, en',
    'FR',
    'Fr',
    'fr en',
    'fr ,  en,',
    'zh_tw';
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