Comment by skylyrac

For a regular DS all you need to do is buy a simple flashcart, copy applications to a microSD card, and you're done, you don't need to replace the firmware. There's even a FOSS flashcart: https://www.lnh-team.org/

On DSi you can replace the firmware and load your applications from the SD slot of the DSi, no flashcart needed.

Also, there are a few C/C++ toolchains for the DS (BlocksDS, devkitARM) and different libraries depending on how low level you want to go with your code. It's very easy to get started.

The 3DS is also pretty easy to hack, and then all you have to do is to copy your applications to the SD card of the console.