Extension methods written in C# can be imported and called with Extension method semantics in VB and vice versa. This is possible since me decorate the assemblies , types and methods in the same manner. Using the Attribute

[AttributeUsage(AttributeTargets.Method | AttributeTargets.Class | AttributeTargets.Assembly)]

public sealed class ExtensionAttribute : Attribute { } 

This should be a special note to developers writing their own compilers or doing IL gen. This decoration allows the importer to precisely identify the classes to import when looking for extension methods. Allowing it to omit unnecessary types and their dependencies when looking for extension methods, which can quickly addup.

So if your doing anything custom be sure to inspect the IL to see if other languages can discover and call your extension methods seamlessly