[NumLock]

у меня от региональных настроек формат ffFixed не зависит, а вот ffNumber зависит от ThousandSeparator.

ffFixed Fixed point format. The value is converted to a string of the form "-ddd.ddd...". The resulting string starts with a minus sign if the number is negative, and at least one digit always precedes the decimal point. The number of digits after the decimal point is given by the Digits parameter--it must be between 0 and 18. If the number of digits to the left of the decimal point is greater than the specified precision, the resulting value will use scientific format.
ffNumber Number format. The value is converted to a string of the form "-d,ddd,ddd.ddd...". The ffNumber format corresponds to the ffFixed format, except that the resulting string contains thousand separators.

Код:

  DecimalSeparator:='.';
  ThousandSeparator:='_';
  Edit1.Text:=FloatToStrF(1455.22, ffFixed, 8, 2);
  Edit2.Text:=FloatToStrF(1455.22, ffNumber, 8, 2);