Beginner friendly is a fairly wide range of things and means different things to different people.
This is the main development kit used by homebrew developers, itself a frontend to various GNU projects (and some extras to make things work on a given console).
However it will necessarily want you to develop in C, assembly (arguably the hardest of the general coding languages/approaches) and maybe C++ but I am not sure how well that works on 3ds offhand (the overhead required vs plain C is considerable so it tends to be a notable leap when that becomes viable for general use on a new otherwise limited system).
It will also come with some nice example libraries, and you can probably also find some open source homebrew to compile and alter and learn a few things that way.
The limit of this is your own coding skill and the console hardware. I don't know that I would necessarily pick the 3ds to learn C on but at the same time if it is going to be the thing that keeps your focus (very few people ever learn for the sake of learning, most of those already being invested in the field as well, and instead need a task they wish to accomplish or will keep their attention) then it is very far from the worst/most complicated/most additional frustrations thing out there. You might face a bit of a learning curve when you move to Windows or Linux or something and want to code in C there as the libraries might be a bit different but you will face different libraries, APIs, functionality and more wherever you go, and the basic "convert this to that" thing that most baseline programs from individuals tend to be are fairly platform agnostic anyway.
Alternatively, and the 3ds is kind of weak on this front compared to the DS in terms of range of things available and popularity of communities using it, is higher level interpreted languages. Easier to code in/learn, easier to manage, easier to port certain things around but ultimately a lot slower so where the C and whatnot approach can easily yield something that could be a commercial game then this... more tricky. There have been high level commercial efforts made using them (Puzzle Quest on the DS for instance is made with Lua) but if you are expecting to be able to make some fun 3d things then... less likely to happen and instead you will mostly be doing more text things, maybe speaking to websites for things, graphics are going to be more like glorified slideshow and so forth.
Python, no idea how well it runs
There were some people attempting graphical programming options but graphical programming for now is more of a novelty or thing you use to teach kids the rudiments of programming.
Lua is in the background of a lot of things and quite popular among gamers -- most of the tool assisted speedrun emulators will be using it, a lot of macro scripts are made with it, several games use it underneath it all, you can make cheat generators and the like easily enough with it. As a general use thing though then it is not that popular.
Python is said to be the glue that holds modern computing together. It is not fast, it has some quirks but if you go into a commercial computing environment then if someone just needed the output of one program/database/whatever to be in a slightly different format to be used in something else it is not guaranteed to be python but you would be foolish to bet otherwise, even more so if it was a "last minute on a Friday evening" type fix.