Is it true that one doesn't "implement a programming language"?
No, that's not true.
You certainly can implement a programming language. For example, Clang, GCC, icc, xlc, PathScale, Metrowerks, pcc, and tcc are all implementations of the C programming language. Oracle
javac and Eclipse
ecj are both implementations of the Java programming language. Zend Engine, Quercus, and P8 are all implementations of the PHP programming language. YARV, Rubinius, JRuby, Opal, MRuby, MagLev, IronRuby, Cardinal, Ruby.NET, XRuby, RubyGoLightly, MRI, tinyrb, Alumina, IoRuby, Smalltalk.rb, SmallRuby, BlueRuby, Carbone, Topaz, RubyMotion, MacRuby, Corundum, and Red Sun are (or were) all implementations of the Ruby programming language. CPython, PyPy, IronPython, Jython, pynie, and Pyston are all implementations of the Python programming language.
I was told that in some comment around here several years ago and I read that actually you do "implement a programming language".
Yes, you can implement a programming language. You don't have to, though: a programming is no less a programming language if it isn't implemented. You just can't run the programs written in it if you don't have an implementation.
For example: the Plankalkül programming language was created by Konrad Zuse in 1942–1946, but it wasn't implemented until 1972. However, in the 30 years in between, it certainly was a programming language.
The background is that I was trying to check if it is true that python is a "programming language" and not a scripting language. I think I found that the difference is meaningless and I have to know how the language is used.
Scripting is a specific kind of programming, so a scripting language is a specific kind of programming language. Asking if Python is a programming language and not a scripting language is like asking if a ThinkPad is a computer and not a laptop.
For example: Perl has a compiler and therefore it is a programming language if the compiler is used. The same goes for python doesn't it?
No. This makes no sense: why would whether or not Perl is a programming language change depending on whether or not I use a compiler? What if you use a compiler for Perl and I use an interpreter for Perl? Does that mean that Perl is at the same time a programming language and not a programming language? How would that work?
It should be obvious by this example, that a language being a programming language depending on how someone chooses to execute programs written in that language is logically non-sensical.
So if one is gonna categorize "programming language", "scripting language", "compiled language" and/or "interpreted language" then you can't know just from knowing the language, you must know how it is used or implemented.
The terms "compiled language" and "interpreted language" are non-sensical: compilation and interpretation are traits of a language implementation, not traits of a language. The language stays the same, regardless of how you or I chose to execute programs written in it.
Again, imagine the following situation: both you and I execute the exact same Python program; I use an interpreter, you use a compiler: now, is Python a compiled language or an interpreted language?
There is no such thing as a compiled language or an interpreted language. The terms make no sense.
"Scripting" is a specific kind of programming, where the program (aka the "script") is part of a larger environment and where the majority of objects being manipulated by the script, the majority of operations being used for manipulating objects, and the majority of data types, are provided by that environment. Also, the lifetimes of objects manipulated by the script are typically independent of the lifetime of the script itself.
Some examples are browser scripting with ECMAScript, where the DOM types, DOM objects, and DOM API are not part of ECMAScript but rather part of the browser, and the lifetime of DOM objects depends on the browser, not the script. Or OS scripting with something like Bash, Perl, Python, or Ruby, where the processes, files, and directories exist independent of the script, and the utilities used to manipulate them are part of the OS, not the language.
Hence I now think that Python and Ruby not necessarily are "interpreted languages", it depends on if you use a compiler or an interpreter.
No, it doesn't depend on anything. They are languages. Period. The term "interpreted language" doesn't make sense. If English were a typed language, it would be a type error.
Again, just think about it logically: IFF Python being an interpreted language depends on whether you use a compiler or an interpreter, what would it be if I use an interpreter and you use a compiler? And if I delete the interpreter and install a compiler, why would the definition of Python suddenly change for everybody on the planet?
It just doesn't make sense.