Recently a Hotfix has been published that have solved a "buggy" scenario in which under some circumstances Web Services Language depended on the Role Tailored client language used on the last session.
This was impacting the date format and was unpredictable so Development decided to set EN-US as language by default for Web Services.
This is the link to the KB Article from where Hotfix can be downloaded:
Unexpectedly the date format in the Microsoft Dynamics NAV web service depends on the RoleTailored client language that you use in Microsoft Dynamics NAV 2009
Therefore builds from 32558 on will include this change (Bear in mind that Platform Hotfixes are cumulative for NAV 2009 SP1/R2 so any Hotfix higher than this will include this change) so please take this into account when applying such hotfix as long as it will have some impact on developments made to Web Services (Specially to date format as EN-US is MM/DD/YY)
Diego García Álvarez
Dynamics NAV Senior Support Engineer