To me if you can competently build a product that you and others can maintain you have mastered a languadge. You may not be an expert in every facet of the lang

To me if you can competently build a product that you and others can maintain you have mastered a languadge. You may not be an expert in every facet of the language but you have demonstrated mastery of it to produce real products. Masters develop a style, and a system to produce results once you have a style and a system that consistently produces results you have mastered it. Lets take art for example and I will use a commonly known artist Salvador Dali who's style was Surrealism of which he is considered a master, he however may not have been good at Fauvism even though he mastered the same tools, but may have never been exposed to the techniques of that particular style. None the less it still leaves him as a master of painting even though he may not have mastered every aspect of painting. Adapting that to technology one may build systems in say Java but never use particular built in classes, maybe they use the Apache classes instead. This is their style and so long as it produces quality results, it can be said that they are a master.