Because the technology that powers the web is based on open standards, there are many choices when it comes to what language a back-end developer should choose. This type of decision generally differs from company to company, and sometimes even within departments at a company.
Modern web sites are typically constructed using a variety of languages. All of the commonly used languages generally have several web frameworks to choose from when making a website. Back-end developers should stay aware of upcoming languages and frameworks, as the industry moves quickly.