The following languages can be used in Sega Dreamcast programming:

Assembly languages

SH4 assembly, as the native machine code for the Dreamcast's SuperH processor
ARM assembly, as the native machine code for the Dreamcast's AICA ARM7DI sound processor

Compiled languages

C, the predominant language for Sega Dreamcast development, via the GCC C compiler.
C++, commonly used and very well-supported, via the GCC C++ compiler.
Objective-C, via the GCC Objective-C compiler. Preliminary support for Foundation framework via GNUstep is being developed. See Andrew Apperley's X account.
- The GNU Objective-C++ extension is available as well.
Rust, via rustc_codegen_gcc and libgccjit, or via gccrs. When using rustc_codegen_gcc, the core and alloc libraries are available, with bindings to KallistiOS being developed. See Rust on Dreamcast for more information.
D, via the GCC D compiler. Early support, see Luna's X account.
Ada, via GNAT. Ada 3D cube demo by Marc Poulhiès.
Fortran, via GFortran. Requires some work to create a cross-compiler, but does work.