When converting for instance a decimal to a string, you use the CultureInfo.InvariantCulture and pass it as an IFormatProvider>
Try one of these:
string valueString = XmlConvert.ToString(value);
string valueString = Convert.ToString(value, CultureInfo.InvariantCulture);
XmlConvert.ToString() is made for XML, so it will keep things closer to the XML spec, such as using "true" instead of "True". However, it is also more brittle than Convert.ToString(). For example, this will throw an exception because of the UTC time:
XmlConvert.ToString(DateTime.UtcNow)
but this works:
XmlConvert.ToString(DateTime.UtcNow, "o")