Length() returns 2x of the real length - so what is the right way?

After the answer to my question, I removed {$mode DelphiUnicode} everywhere and I still use String everywhere. As I got, String means AnsiString if {$mode DelphiUnicode} is not used.

And I have a code:

Var
  CmdLineArgs: CommandLine.Args;
  infile: Text;
  line: String;
  output: Boolean = True;
Begin
  CommandLine.Parse(self, CmdLineArgs);
  WriteLn(CmdLineArgs.InputFile);

  AssignFile(infile, CmdLineArgs.InputFile);
  Try
    Reset(infile);
  While Not EOF(infile) Do
    Begin
    ReadLn(infile, line);
    If output Then WriteLn(line, ' ', Length(line));
    End;
  Finally
    CloseFile(infile);
  End;

The passed via command line options file is UTF-8. And I see that Length(line) returns everytime 2x length to the real symbols number. But if I turn on the {$mode DelphiUnicode}, then it shows the correct length (as I said, I turned off it because of this). It means that AnsiString's Length returns bytes number, not codepoints number. So, the question now is: what is the correct pragmatic and modern way to work with national languages (in files, in constants, everywhere) in FPC - I prefer UTF8 encoding? I am with 3.2.2 version.

EDIT:

Args is just an object with a processed file path:

Type
  Args = Object
    InputFile: String;
  End;