ひゃほい。
function MapStringW(const Source: WideString; dwMapFlags: Longword): WideString;
var
　Len: Integer;
begin
　Result := '';
　Len := LCMapStringW(LOCALE_USER_DEFAULT, dwMapFlags,
　　　　　 PWideChar(Source), -1, nil, 0);
　SetLength(Result, Len-1);
　LCMapStringW(LOCALE_USER_DEFAULT, dwMapFlags,
　　PWideChar(Source), Length(Source)+1,
　　PWideChar(Result), Len);
end;
function KataToHira(const Source: WideString): WideString;
begin
　Result := MapStringW(Source, LCMAP_HIRAGANA);
end;
function HiraToKata(const Source: WideString): WideString;
begin
　Result := MapStringW(Source, LCMAP_KATAKANA);
end;
function ZenkakuToHankaku(const Source: WideString): WideString;
begin
　Result := MapStringW(Source, LCMAP_HALFWIDTH);
end;
function HankakuToZenkaku(const Source: WideString): WideString;
begin
　Result := MapStringW(Source, LCMAP_FULLWIDTH);
end;