The policy for SOAP Web Services in NAV 2013 states that they always run in EN-US culture.
In NAV 2009 it was reported a buggy scenario that was solved in build 32558 and upcoming ones (remember)
However after collecting some feedback a workaround was built for NAV 2009 (remember 2)
This workaround was temporary as product team did not want to introduce a breaking change in a Hotfix without a way to revert to the original behavior.
Therefore now in NAV 2013 it has been decided to set EN-US as the default fixed culture for SOAP Web Services, the aim behind this decision is that we want to gurantee that Web Services have a fixed culture insensitive interface so any system can achieve a predictable communication with NAV.
I hope you find this Information helpful.
Diego García Álvarez
Dynamics NAV Senior Support Engineer