Description:
------------
WDDX does locale aware conversion from double to string. Unfortunately it does not support locale aware conversion back to double. Anyways: I would suggest to disable locale aware conversion and always use a point (.) as decimal seperator.

Test script:
---------------
setlocale(LC_ALL , "German");
$x = Array("XYZ"=>5.1);
$y = wddx_serialize_value($x);
var_dump($y);
$z = wddx_deserialize($y);
var_dump($z);
echo ($x == $z) ? "OK" : "FAIL"; 

Expected result:
----------------
string(120) "<wddxPacket version='1.0'><header/><data><struct><var name='XYZ'><number>5.1</number></var></struct></data></wddxPacket>"
array(1) {
  ["XYZ"]=>
  float(5.1)
}
OK

Actual result:
--------------
string(120) "<wddxPacket version='1.0'><header/><data><struct><var name='XYZ'><number>5,1</number></var></struct></data></wddxPacket>"
array(1) {
  ["XYZ"]=>
  int(5)
}
FAIL