I wish to create shareware software that contains a registration algorithm. I am looking for a programming language, that cannot be easily decompiled into readable code. F
If you're lookig "for something that can be only decompiled into assembly", that essentially means that you want to use a language that gets compiled (or assembled) directly into a native code executable.
The usual prime suspects then are C, C++, Delphi, VB6. Of course, also assembly meets your criteria, although I doubt you'd want to write any project of decent size in it.