So why is it easy for a foreign student to learn English, but so much more difficult the other way round?
I will add to Lurking's post.
For a vast number of people around the globe, speaking English is an academic and economic necessity. You may not be able to get a place at university without fluency in English; you may not be able to get a job in your civil service without fluency in English.
I worked in a country where if a student did not pass the FCE, their employment opportunities were pretty much limited to agricultural work -- no matter how intelligent or able they were in other subjects. It was the passport, so there was a lot of motivation to learn the language.
Now, because English is the language to learn for so many people around the world, there is an enormous market for English language learning materials and resources, which means the business opportunities in ESOL materials are extraordinary, which means huge amounts of English language learning materials are produced and sold.
This is important because the wealth of educational material produced to guide learners from A1 to C2 level makes it easier to learn and practice the language. There are multitude of text books, online resources, dictionaries, CD-Roms, colouring books, software programmes, flashcard packages ... all catering for the market.
And this works hand in hand with global access to English language media products: music, films, TV series, books, cartoons, websites, even the English language commentary on premier league matches. And because the English language media industry is so huge, these products are fairly cheap.
The support these media products gives an ESOL learner is incredible. English becomes an environmental feature that can give a learner near constant exposure. You don't want to study English grammar today? Just turn on the radio and, ten to one, it will be a song by Beyonce. You and your teen friends want to go out? Go see the latest Bond film at the local cinema; it will probably be subtitled in your language, but that still means you are constantly hearing English for those 90 minutes.
By contrast, this wealth of material and exposure simply doesn't exist for an English speaker wishing to learn another language, and that is what makes it so hard.