So you have to treat them like they are very special.  Give them everything they want, almost as if they were ordering a custom application.  You may have to implement special features just for them.  You may have to give them substantial discounts.  You should visit their site and meet them in person.  You may have to install your product for them. 

But mature languages can't change easily if at all. This situation is described in theory of frozen evolution. Basically the evolution of language is similar of the evolution of a new specie. Evolution happens in short burst when the new language(specie) is created. In this period language design is plastic, there are few if any users, little amount of code and libraries and the community consists of enthusiasts. This short period of plasticity is fallowed by a long period of stability. The evolution of the language is frozen. There are few tweaks here and there but nothing radical. And nobody , not even the original creator could change that.