@IvenBach FWIW, I have replicated what you are experiencing. But, if I add the new .resx file directly under Rubberduck.Core and NOT in a subfolder of Rubberduck.Core - the build succeeds. Unfortunately, I do not know what this means, but maybe it can be a piece of useful info to someone who does.
@IvenBach Here's a path the allows the compile to succeed. Others can comment on whether or not it is valid.
After inserting the new resource file in Rubberduck.Core, try to compile...it will fail (as we know). Open Rubberduck.Core.csproj find and comment out this EmbeddedResource element:
The part to comment out (In my message) is near the top of the .csproj file. (I am not showing the entire file) The xml in your message IS added to the .csproj when you add the .resx and attempt to compile and should remain as-is. Sorry, I am away from my machine so I am going off memory at this point.
Glad it now compiles...I think the added lines is what points the compiler to the newly inserted .resx and associated .cs file. Not finding the .cs file was cause of the previous build failures.