I have a Macintosh SE FD HD running Apple System 6. Is there a Terminal application? What programming language(s) can I run on System 6?
There is no terminal application for System 6, at least not out of the box. There is Macintosh Programmers Workshop and a couple of other oddities around, as well as things like telnet clients for connecting to other computers.
While I don't know all possible programming languages, MPW will give you a command line, C, C++, and Pascal. Other languages I've seen were Chipmunk BASIC, Perl, and other development environments that were common were Think C and Metrowerks CodeWarrior. Here is a random webpage about it.