Correct slovak locale naming is sk_SK not only sk, this is bug in locale settings in sqmail, I think.

functions/i18n.php

@@ -119,9 +119,8 @@
    $languages['tw']['CHARSET'] = 'big5';
    $languages['tw_TW']['ALIAS'] = 'tw';
    
-   $languages['sk']['NAME']     = 'Slovak';
-   $languages['sk']['CHARSET']  = 'iso-8859-2';
-   $languages['sk_SK']['ALIAS'] = 'sk';
+   $languages['sk_SK']['NAME']     = 'Slovak';
+   $languages['sk_SK']['CHARSET']  = 'iso-8859-2';
 
    // Decodes a string to the internal encoding from the given charset
    function charset_decode ($charset, $string) {