in src/Microsoft.NuGet.Build.Tasks/ResolveNuGetPackageAssets.cs [467:476]
private bool IsFileValid(string file, string expectedLanguage, string unExpectedLanguage)
{
var expectedProjectLanguage = expectedLanguage;
expectedLanguage = expectedLanguage == "C#" ? "cs" : expectedLanguage;
unExpectedLanguage = unExpectedLanguage == "C#" ? "cs" : unExpectedLanguage;
return (ProjectLanguage.Equals(expectedProjectLanguage, StringComparison.OrdinalIgnoreCase)) &&
(file.Split('/').Any(x => x.Equals(ProjectLanguage, StringComparison.OrdinalIgnoreCase)) ||
!file.Split('/').Any(x => x.Equals(unExpectedLanguage, StringComparison.OrdinalIgnoreCase)));
}