Marek,

   By resource, what do you mean?  I ask because WPF allows for embedding
resources within XAML files, and you could actually embed XML in the XAML
file as a resource.

   Or is this a resource file that you are adding to the project, and then
acessing that?  If this is the case, then the designer should have added a
type-safe wrapper to access the resource through.

   Or are you just adding the file to the project and setting the Build
Action to "Embedded Resource"?  If this is the case, then the first call
should work, assuming that the default namespace for your app is "MyApp".
You need to check the properties for the project and on the Application tab,
look at the "Default Namespace" as that is what needs to preceed
"Langages.xml".