If a class is mapped with BsonType.Double but the underlying type read from the database is Decimal128, the .NET Driver will attempt to convert the Decimal128 value to a double using Decimal128.ToDouble(). The problematic lines are:

var stringValue = d.ToString();
return double.Parse(stringValue);

Decimal128.ToString() is hard-coded to format the string using a decimal point. double.Parse(stringValue) does not specify a locale and therefore parses it in the locale of the current thread. In locales such as FR that use a comma as a decimal separate, double.Parse(stringValue) fails. We should be parsing using the CultureInfo.InvariantCulture as we did in CSHARP-1943 for a similar issue.