The built-in kendoDropDownFilter directive normalizes text using String.prototype.toLowerCase() when caseSensitive: false. This is locale-insensitive and produces incorrect results for languages with non-standard case-folding rules, most notably Turkish.

The correct fix is to use String.prototype.toLocaleLowerCase(locale), which respects locale-specific case rules. The DropDownFilterSettings interface should expose a locale option for this:

filterSettings = {
  caseSensitive: false,
  operator: 'startsWith',
  locale: 'tr-TR'   // ← proposed new option
};

Affected components: kendoDropDownFilter directive (used with AutoCompleteComponent, ComboBoxComponent, MultiColumnComboBoxComponent, DropDownListComponent, MultiSelectComponent).