By default Aspose.Words uses the current thread culture to perform parsing and formatting when calculating field values during field update and mail merge. There are also options provided in the form of the FieldOptions class which allows further control over which culture is used during field update. Secondly, you can use the following code in order to change the culture of thread:
CultureInfo currentCulture = Thread.CurrentThread.CurrentCulture;
Thread.CurrentThread.CurrentCulture = new CultureInfo("en-US");
Please let me know if I can be of any further assistance.