language-agnostic
By now I work with asp.net and C#. I have done a decent work in Java as well. I am planning my career in such a way I should be language-agnostic someday. What are the things that I need to learn?
First would OOP paradigms as its speaks about the Class design. Are there any others?
In the VIEWPOINT section of Communications of The ACM
If you're interested in practical programming, the proceedings of ACM and the likes is the last source you want to read. These are often [pseudo]-scientific publications with no application in the real world. These are quite often unorthodox opinions made for publicity, for the writer to differentiate himself from the crowd and promote his own person.
I claim that the use of OOP is not as prevalent as most people believe, that it is not as successful as its proponents claim, and, therefore, that its central place in the CS curriculum is not justified.
I tend to disagree with your point. OOP is widely spread and works just fine. By the number OOP-based projects have likely surpassed developments done with other strategies (let's speak about modern time, 15-20 years).
However OOP is not a silver bullet. It works for some developments, doesn't work for others. Just like any other approach.
But one thing I need to mention is that a curriculum should communicate knowledge of different approaches. If it's OOP-based, it's wrong. If it's FP-based, it's wrong. It should cover them all or don't touch this topic altogether.
P.S. Why care about what is dominant and what is not? Just take what is suitable for the project at hand and leave the numbers to "researchers".
You cannot do that because some languages implementations could have an ABI different and incompatible with the C one.
On current systems, C is very common and most (but not all) language implementations have an ABI and calling conventions compatible with the one from C. Notice that it is a property of the implementation (your C compiler and your operating system), not of the programming language.
Also, some languages have different and incompatible views on control flow (look into call/cc and tail-calls in Scheme, Goroutines in Go, or backtracking and cut operator in Prolog, CLIPS rules, concurrent actor languages, ....). I'm not sure you'll be able to design something which easily fits all of them (and that could impact the FFI requirements of implementation of these languages).
If using C++, beware of name mangling and of exception handling. (e.g. C longjmp is not friendly with C++ exceptions). Dynamic loading facilities like dlopen and dlsym are relevant to name mangling. So prefer an API using extern "C" functions. libgccjit could be inspirational (it is coded in C++ but has a C friendly API) and perhaps useful (you could consider runtime generation of glue code).
Memory management (notably with garbage collection) is also an issue. Study for examples foreign function interface of Ocaml and of SBCL and of Lua and of Guile. Look into libffi. Some languages also want serialization or persistence facilities (even for foreign data). Read also about dynamic software updating.
You could provide some reflection facilities (e.g an API to query your API, e.g. like GTK introspection). You might try to provide a generic closure mechanism like in GObject-s.
You could use (or customize or adapt) code generators like SWIG. You might consider compiler plugins (e.g. GCC MELT extensions).
If you have a wide and complex API (hundreds of public functions or data types) consider at least providing a machine readable form of it, e.g. some database or XML or JSON from which automatic glue code could be generated.
NB. Better make your stuff free software. You might get outside contributions and you'll need outside help to interface your thing with many programming languages, including some that you don't even know. Open source is a good way to counter leaky abstractions (since you and other contributors can dive into the implementation source code). Given the variety of programming languages, you will need outside help (as soon as your API is successful)
I asked several times the OP to give much more context and motivation, and to explain the domain for which the library is written, and the context in which that library would be developped. Sadly I've got no answers at all.
The applicative domain (e.g. high speed frequency trading, software for dentists, selling machine, weather forecast, image processing, speech recognition, word processing, social network, static source code analysis ...) matters practically a lot, because it defines what programming languages are likely to be considered and what programming paradigm is probably used. For instance, an image processing library is very unlikely to need to be interfaced to Agda or Coq, but probably would be used from C or C++ applications. Business software is often written in Java (and older ones was in Cobol) so needs to be interfacable to JNI or JVM. Static source code analysis software is very likely to use some theorem prover (so Ocaml or Common Lisp or Haskell is important to them, and they need to be garbage-collection friendly). And so on. Sometimes, inter-process communication (including RPC, MPI, web services, RESTful applications) can be used, but at other times it is not efficient or not convenient enough. Details and context are very important and we don't know them.
The question mentions "desktop software", but that just means today something having a native (non-Web) GUI used with a mouse and a large enough color screen (the dentist software, the software for managing my bank accounts, your favorite game software, the word processor, a web browser are all desktop software, but their main commonality is just having a GUI). Using Qt (a very powerful GUI cross-platform toolkit for C++) facilitates the development of such things. And quite often a library used on desktop (think of XML or JSON libraries like Xerces and JsonCPP, machine learning libraries like TensorFlow, HTTP client libraries like libcurl, numerical computation libraries like BLAS or GMPlib, etc...) don't even care about GUI (but it could care about response time -having functions returning in less than 0.4 seconds- and thread friendliness), because the application (not a reusable library) would care about GUI itself.
The question mentions plugins, but these are simply compiled software components which get dynamically loaded at runtime (thus increasing the virtual address space of the process loading them), using e.g. dlopen on POSIX (and probably LoadLibrary on Windows, which I don't know at all). Notice that the JVM don't use (Java-coded) plugins but dynamically loaded classes, and speaking of plugins don't make sense with JavaScript or with Common Lisp (or most languages having a compile or eval primitive). So the mention of plugins don't means much (both Firefox browser and GCC compiler accept plugins, for very different reasons).
Without a lot more additional details (including applicative domain, context, motivation ...) the current question don't make any sense. I offer to delete my answer if that helps in closing the question in its current form.
PS. I don't know and never used Windows (but I use Unix since 1987, and Linux since 1993), but that is an unimportant detail.
Best Answer
To be language agnostic you need to have experience in all of the common styles and types of languages.
Also:
Some typing styles:
Experience of strong vs. weak typing is also useful.
Some different runtime styles:
Lower level stuff:
On top of that I would say you need experience of some concurrent programming and something event driven. You should probably also make sure you know something about the various domains such as web programming (client & server), rich client development/desktop, games. You might also want to learn about embedded programming, or dedicated hardware (like games consoles), and mobile development is becoming an increasingly relevant domain.
Others have also mentioned that it's worth getting some experience of Generic programming and Meta programming approaches.
When you learn these paradigms avoid just learning the syntax and writing in your old style. I've seen many C# devs write JavaScript as if it's statically typed. Don't do this, try to learn the language paradigms and embrace them.
If you've done all of this, the differences between languages will become largely syntactical so switching will become a fairly simple exercise of learning some new syntax.
Don't forget though that modern programming is almost always dependant on a framework, so familiarising yourself with the common and popular frameworks for each language you learn is also critical. Knowing C# is irrelevant without .net.