You are absolutely right. For my current client I had to pick up C# very quick. Before that I had only created a couple of "toy programs" to play with the language. But I already knew a lot of the concepts behind C#: Inheritance, delegates, properties, lamda expressions, ... so learning the new language was really easy and I completed the first programming tasks within the first week.

Unfortunately some recruiting agents only do simple pattern matching and they won't forward you CV to a client when you do not have $language_name skills :(