Is it easier to attain the four language skills (listening, speaking, reading and writing) in language A than in language B if neither of them is your first language?
I think that some languages are harder to learn. I have traveled a lot around the world, and I have also learnt different languages in the past. Some languages may be hard to learn if, for example, the alphabet is much bigger, there is a greater variety of sounds (phonemes), or the grammar structure has more variations. For example, many people think that the grammar in French is more difficult than the one in English. But some people would find English difficult because of the inconsistencies in pronunciation. It probably depends on the learner's talent as well and where his/her strength is, e.g. if they have a good ear or oral expression. Or if they are logical thinkers, they may absorb grammar more easily. Obviously, it also depends on what the learner's first language; for someone whose first language is English, it may be easier to learn German, since these languages are in the same linguistic family. Or a native Portuguese speaker may learn Spanish more easily. Some languages also have a reputation for being hard to learn, such as Arabic, Chinese, Japanese... :)