A developer is any person who "develops" any application, software or website, through the use of programming languages.

Some of the languages known and used for this are: HTML, Javascript, SQL, PHP, ASP, XML among others.
The previous languages refer to languages used in website development.

Code C, Code C++, Java, Assembly, among others.
The previous languages refer to languages used in software development.

In most cases, a developer is recognized for using different programming languages, however this can sometimes change a bit with block programming.