How i can add resource for additional language, under existing resource file?
See picture from Solution explorer:

If I try add new one I get this:

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When adding a resource file, Visual Studio will add it at the folder level, as you have seen.
In order for you to get it displayed as a sub-node, you will need to edit the
.csprojfile directly (you can right click the project, select “Unload Project” then right click again and select “Edit .csproj” – when done, reload the project via the right click. Alternatively, open the.csprojfile in any text editor).You will an entries like this:
You will need to change this to:
This will put both
Error.cs.resxandError.cs.Designer.csunder theError.resxnode.There is no way to directly do this in the solution explorer.