Creating bots and Editing clients

Whats the best way/language to create bots in rotmg, features such as reading chat, reading player inventory, reading any sort of info from chat, etc.

And also whats the best program to edit clients, change some code, change sprites, small edits and compilers, etc.

Anything external - C# (since K-Relay, which you probably wanna use unless you're going for something clientless, is written in C#)
Anything internal - ActionScript (since we're talking about Flash)
Editing SWF - JPEXS for just about anything, optionally RABCDAsm for hacking the in-game scripts

If you want to make any kind of a bot that directly works with the game, I'd really suggest using K-Relay. It gets updated regularly and it allows you to do nearly anything. Writing a plugin for K-Relay is also fairly easy once you become familiar with the RotMG packets and data structures.
K-Relay also has a fairly good "how to get started" guide for those who want to make their own plugins, so that could be rather helpful to a newcomer.

If you want to go clientless, which is what most of the chatbots and multiboxing utilities probably use, you could try nrelay. It has an awesome documentation, but I'm not really sure what state of development it currently is in.

anything external, ie relays or clientless bots can be done in just about any language, so basically whatever you're most comfortable in.

as far as editing a swf file goes, I wouldn't suggest doing anything but small edits with jpex, you really want to decompile and work with an actual source, which leads me into my next thing

as far as making bots go, if you're more leaning towards the clientless route you're going to basically be doing everything with packets and can use a pretty wide variety of languages.

if you're going for just a client bot then you're only option is to decompile and build your bot within the actual client, you have access you everything you listed and soooooo much more.

As far as software goes, you're going to want a decompiler, probably multiple, I use trillix and as3 sorc personally, you're going to want an an ide to work with the source of the game, I personally use flashdevelop, I've seen other people around here use intellij but I didn't like it. Those are really the main things needed, along with a decent amount of just general knowledge in programming and game reversing.

Thank you both for the responses, I'll look into the things you've listed <3

I have a friend making clientless bots in C# right now which seems to be going well.