Fortran is still used in science and engineering, but its syntax and structure are very different from those of current languages. This can be hard for people who are just learning it.
Lisp has a unique and often challenging syntax due to its heavy use of parentheses. It's known for its powerful metaprogramming capabilities.
C++ is used a lot, but it is known for being hard to understand and having a lot of features. It can take a long time to learn how to speak a language well.
Haskell is a completely functional programming language that places a heavy focus on mathematical concepts. Its unique syntax and the requirement to think functionally can be difficult for many programmers.
Prolog is a logic computer language that is very different from most other languages. It is mostly about symbolic thinking and programming based on rules, which can be hard to understand.