If I'm reading the docs and examples correctly, I think you should be able to do something like this (untested):

#!/usr/bin/perl
use warnings;
use strict;
use Tk;
use Tk::JComboBox;

my $mw = MainWindow->new;
my $lang1;
my $lang2;

my @choices = qw(en fr de it sp);
 
my $drop1 = $mw->JComboBox(
    -textvariable => \$lang1,
    -choices => [@choices],
    -entrybackground => 'white',
    -background=> 'white',
    -disabledbackground=>'grey',
    -autofind => { -enable  => 1 },
    -selectcommand => \&changeDrop2
)->pack();
    
my $drop2 = $mw->JComboBox(
    -textvariable => \$lang2,
    -choices => [@choices],
    -entrybackground => 'white',
    -background=> 'white',
    -disabledbackground=>'grey',
    -autofind => { -enable  => 1 },
)->pack();
    
MainLoop;

sub changeDrop2 {
  my ($drop1, $selIndex, $selValue, $selName) = @_; 
  my @restricted = grep { $_ ne $selValue } @choices;
  $drop2->configure(-choices => [@restricted]);
  $mw->update;
}
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