hi,
look at these posts:
NAV 2013 SOAP Web Services on a multilanguage environment
blogs.msdn.com/.../nav-2013-soap-web-services-on-a-multilanguage-environment.aspx
community.dynamics.com/.../nav-2013-soap-web-services-on-a-multilanguage-environment.aspx
hotfix
2876779, 7.0.35212
September 2013: Platform Hotfixes for Microsoft Dynamics NAV 2013
"it is not possible to change Web service culture"
nav.dk/.../september-2013-platform-hotfixes-microsoft-dynamics-nav-2013