I was present at the 2nd International Radiation Research Congress in 1962, and one evening I had a fascinating discussion with one of the translators who were working there at the time. Nobody knew what nationality this guy was (the majority opinion was Greek, but nobody was sure), but I was assured that he spoke all the major European languages flawlessly, and a host of others with a skill that ranged from very good to passable.
He said that English was the easiest language that he knew in which to communicate (i.e. exchange basic information) effectively and among the most difficult to speak really well. In his opinion, one of the bigger stumbling blocks was its use of phrasal verbs (i.e. a verb closely followed by at least one preposition &/or adverb). These form a semantic unit whose meaning cannot usually be deduced from the meaning of the individual words. For example, if you "look up to" someone, you respect him (or her) and consider him (or her) in some way superior; you do not mean that he/she is above you in the Earth's gravitational field by, say, standing on top of a ladder – the phrase you would use there would be to "look up at" him, and that is not a phrasal verb.
And that's before you get on to pronunciation and spelling, and English's confusing custom of pronouncing the same group of letters differently depending on the word they are in. For example, I know of 8 ways of pronouncing the letters "ough": e.g.
though (as in "so"),
through (as in "boo"),
thought (as in "sort"),
thorough (as in "the"),
rough (as in "stuff"),
cough (as in "toff"),
bough (as in "cow"), and finally
hiccough (as in "sup"),
and there may be one or two more, although I cannot think of them at the moment.