MapPoint.X and MapPoint.Y are both double, so if your system is set for a "comma" decimal separator your best bet might be to convert them both to a string, and then replace the "comma" with a "decimal" since that is what the Google URL is expecting.
Something like this might work:
string yString = coords.Y.toString().Replace(",", ".");
string xString = coords.X.toString().Replace(",", ".");
And then replace coords in the URL with yString and xString respectively. I would think that would work, but I haven't actually tried it.