Imagine this:
Two Resource files named Strings.resx and Strings.de.resx.
The first (neutral) contains english content, the second german content.

If you ask youself over and over again why the f**k the ResourceManager returns the neutral language even you use the specified language “de”, look into your AssemblyInfo.cs file.

If there is a [assembly: NeutralResourcesLanguage("de")], the ResourceManager from this Assembly thinks you want to get the neutral language if you ask for “de” explicit.

This attribute is used to overrule the neutral settings to a specific one.

Deleting this from your AssemblyInfo.cs can help you realy much in this case.