I am a web developer, but I always use PHP. Honestly, I know some basics of Java and Python that I studied at university, but in my area seems like that all companies develop for Microsoft. So, I am thinking to learn C# but I have a Mac. Otherwise, there is the C/C that I never studied.
I don't have a particular project in my mind, maybe a simple browser, but I am not sure.
All of them. It's more important to learn paradigms than to learn just a language. Each language will forcibly teach you new paradigms which you can then use when writing in other languages. Knowing both Python and C, for example, makes me a better programmer in both languages. It also lets you choose which language will most easily meet the needs of each task, and even open you to the possibility of mixing languages in different areas of one project.
Adding on to this, it is also good to learn more than just your standard programming languages. Having a good, working Unix knowledge goes a long way in making your life easier. Tools/languages that I would recommend getting intimate with are awk, sed, grep, ack, make, and general shell scripting. These tools have helped make my life as a programmer so much better, and saved me countless hours of repetitive tasks. Also, keep in mind that you don't have to learn everything mentioned in this thread all at once. You can spend years learning them. :)
"Software development" is a really broad category. Is there something specific you have in mind? If not, you might look on job listings and see what companies are looking for, and if any of those jobs appeal to you. You should figure out what you want to do before you worry about which language to learn; the first decision informs the second. If you don't care about job opportunities, maybe dabble in a few languages and see what jumps out at you, but you should get a sense of what people are writing stuff in. Browsers are generally written in C++, but there are all sorts of things they deal with (rendering engine/js engine, ui) that aren't necessarily C++. If that's the direction you want to go, you might start there. A browser is actually pretty heavy duty for a "screwing around" project; at this point browsers are for all intents and purposes operating systems sitting on top of operating systems. Anyway, if you're particularly interested in browsers you will likely be write that in C/C++, but I don't know enough about it to tell you where to begin apart from picking up the language. If you aren't specifically worried about finding a job, you might want to check out Go; all the kids love it. Before investing time and effort in that you might look what opportunities that is going to give you, though, if that matters. C++ is probably a pretty safe bet in that respect. C# can be written on mac, linux, or windows just by using an IDE that supports it. Monodevelop, or something like it. http://www.monodevelop.com/ Until you have a better idea of what you want to do, I don't know that anyone will be able to help you too much. Look around on the interwebs and see what jumps out at you.
Could you go into more detail on what kind of goals you have in mind? I don’t think your question can be answered on a general basis, but highly depends on what you want to achieve. Do you want to be better prepared for the companies in your area? Or make your ideas a reality in a fast and convenient way? Is it mobile, web, industrial, research,… development you’re interested in? Depending on your answers, the recommended programming language(s!) will differ.
I don't have any particular project or goal in mind. I don't know what to build and I know that a browser maybe is a very difficult project. Maybe also a photo management app is good, or an uploader of photos for Flickr. In job listings the most required is Java and then C#, but I am not looking a new job. I would like to learn software development because it seems to me more complicated and fascinating than building a website.
I found it useful to keep a notebook of ideas that I add to as they come along. No idea is too trivial or too stupid to go into it - they might be the tip of the iceberg of a good idea, especially if a theme keeps recurring. I now have 80+ ideas, ranging from simple programming exercises to fully fledged projects. Do this, and then pick one and implement it in a few languages.
All of the PHP, Java, Python, C, C# that you mention are imperative languages. If you want to expand your horizon you could look into some other paradigms. Personally I'd recommend taking a look at Smalltalk, which is a "pure" object oriented language unlike java, or a functional language such as Haskell (if you're into web development perhaps a dialect such as Elm might be more interesting; but Elm is kind of young and small). You can also learn a multi-paradigm language such as Racket. Coming from world4ch, I'll give you a standard response and tell you to read SICP.
You should go for a well-managed language with lots of well-established libraries for accomplishing the most common tasks. You want a language you can edit in one of JetBrains' IDEs (like IDEA, PyCharm, RubyMine, etc). Don't go for anything too esoteric. Be pragmatic. Don't use flimsy shiny-toy tools like NodeJS and MongoDB. Your programming language is a tool for getting shit done, unless you live in Academic La-La Land where you don't have to worry about producing anything of value because your salary is forcefully extracted from other people. Avoid the Microsoft stack. They just want to hook you into their ecosystem so that you'll contribute to their revenues. Of course, every language comes with its own set of trade-offs. A couple of years back, I would have recommended Python. Now I'm not so sure, because they're still having trouble with the v2 --> v3 transition, and they still don't have "actual concurrency". It's a very well-managed ecosystem with lots of good libraries though, steered by sensible people. With version 8, even clunky old Java has turned into a pretty good language. It's got the best VM out there, any library you could ever need, and it's well-managed too. It comes with quite a lot of historical baggage though. Clunky old, overly complex frameworks that still depend on pointless ancient turd-nuggets like commons-<whatever>.jar even in 2015. The mind boggles. Someone else recommended Clojure. The man has good taste. Use Facebook's React. It's the correct way to do Web UIs.
If you're just looking to develop software, pick X language / framework of the month (i.e. node.js for web, C# for Windows applications (this may be expanding with Mono for OS X / linux), C for low-level control, etc). If you're just looking to learn about the diversity of languages, there's a number of styles that you can play around with. Learn about type systems and compare how you would design an interface in strict vs. dynamic vs. duck-typed systems. Try out manual memory management in C/C and learn why garbage collecting is so much less of a headache. Try writing programs with completely deterministic run-time. Compare syntax between python and lisp. Investigate how even the simplest programs are translated into assembly. Try Scala / Racket for alternative ways to interface with Java frameworks. Your question is pretty open-ended, so it's hard to provide more than a laundry list of different tools and applications...
If you aren't immediately looking for a new job, then I'd avoid the Microsoft stack, frankly, especially if you are a mac user. The linux-based development world tends to have a lot more free tools and easy options, and a much lower learning curve for people not already running Windows. One option is to get a copy of Seven languages in seven weeks and work through the languages listed - maybe that will help clarify your mind as to what kind of thing you want to learn in more detail. My personal favourite language is Clojure, but I've been a coder for a long time, so I can't really say how it would be for a learning language for you. But it's a great mix of a solid functional programming language, with a pragmatic approach to getting real-world stuff done, and a big community behind it. </plug>in my area seems like that all companies develop for Microsoft
and elsewhere
I am not looking a new job. I would like to learn software development because it seems to me more complicated and fascinating than building a website.
Unity is alright, I think when I did work in Unity I just used JavaScript for it. It's nice if you're a good 3D artist or have one working with you, but when I want to do something on my own, I just use AS3 and AIR. Hopefully AS3 has more life in it yet, I don't wanna learn yet another language unless necessary. :|
In general I agree with what everyone else is saying, but I don't know how important it actually is for you, since you do know how to program. If I were you, I'd think about just straight applying to some jobs. That said I'd not do it at dice or monster or any of those huge, generalist sites as those posts generally go through HR first and they aren't familiar with programming enough to not enforce strict interpretation of requirements.