You want to know the best way to mark your main assembly for a given locale.
Use the NeutralResourcesLanguage attribute on the main assembly. This will inform the resource manager about the language for neutral resources that are embedded in the main assembly.
Using this attribute speeds up resource loading when the current user’s locale matches the culture used for resources in the main assembly. Don’t use the AssemblyCultureAttribute in the main assembly. The main assembly should only contain neutral resources (for example, strings and images related to the default language).
// This particular assembly contains resources for the US English culture. [assembly: NeutralResourcesLanguage("en-US")]