I understand that this is an old question, however, in case it helps someone else to find here →>
You need to set the .resx assembly action to "Embedded Resource" to work with Data Annotations, as in this example. When you add .resx to the App_LocalResources folder, the assembly defaults to Content.
In addition, in the above example, you added it to another project, so make sure the access modifier is also set to "Public" (double-click the .resx file to open the file, and the access modifier is located at the top of the editor window).
Also make sure you add a link to the resource project from the main application.
magritte
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