How to compare Unicode strings in Javascript?

You can use Intl.Collator or String.prototype.localeCompare, introduced by ECMAScript Internationalization API:

"Ł".localeCompare("Z", "pl");              // -1
new Intl.Collator("pl").compare("Ł","Z");  // -1

-1 means that Ł comes before Z, like you want.

Note it only works on latest browsers, though.

Here is an example for the french alphabet that could help you for a custom sort:

var alpha = function(alphabet, dir, caseSensitive){
  return function(a, b){
    var pos = 0,
      min = Math.min(a.length, b.length);
    dir = dir || 1;
    caseSensitive = caseSensitive || false;
    if(!caseSensitive){
      a = a.toLowerCase();
      b = b.toLowerCase();
    }
    while(a.charAt(pos) === b.charAt(pos) && pos < min){ pos++; }
    return alphabet.indexOf(a.charAt(pos)) > alphabet.indexOf(b.charAt(pos)) ?
      dir:-dir;
  };
};

To use it on an array of strings a:

a.sort(
  alpha('ABCDEFGHIJKLMNOPQRSTUVWXYZaàâäbcçdeéèêëfghiïîjklmnñoôöpqrstuûüvwxyÿz')
);

Add 1 or -1 as the second parameter of alpha() to sort ascending or descending.
Add true as the 3rd parameter to sort case sensitive.

You may need to add numbers and special chars to the alphabet list