If a programming language supports the SH2 (Hitachi name) or J2 (Open-source name) processor as a build target / compiler target, you could theoretically write a program for the Saturn in that language, as the program target is the SH2's [it goes to their memory].
However, you can only practically use the C program language because SGL, SBL, and Jo Engine (as well as all other available extensions/snippets) for the Saturn are in C (as well as the process of making iso's set up for C). If you wanted to use another language, you would still ultimately have to go back to C to translate the hardware access functions back into a usable form (unless you wanted to find the Assembly function tables for everything but the SH2's/J2's.. which you still might need.
In other words, yes only C. And not even all of C, only some of it, and some C functions are done better by SBL or SGL functions (dividing numbers when decimal points are desired, for example, can work more desirably using FIXED data type with slDivFX). Jo Engine also has a number of handy shortcuts, look in math.h.
For more information on this, you can find SGL/SBL documentation around the net. I think some of it even on Jo website?
