String.Format consider locale or not?

You want to use CultureInfo:

value.ToString("N", new CultureInfo("vn-VN"));

Using String.Format:

String.Format(new CultureInfo("vi-VN"), "N", value);

Since you're in Hanoi (from profile), I used Vietnam's code, which is vn-VN.

This works. The formatted value is 123.456,789 which is correct per es-ES

   IFormatProvider iFormatProvider = new System.Globalization.CultureInfo("es-ES");
   var value = 123456.789001m;

   string s = value.ToString("#,##0.000", iFormatProvider);

   string s2 = string.Format(iFormatProvider, "{0:#,##0.000}", value);

   FormattableString fs = $"{value:#,##0.000}";
   string s3 = fs.ToString(iFormatProvider);

Note that the , and . are using a 'standard' en-US style, but .ToString() and string.Format() with a format provider does the right thing.