I think English is actually not that hard to master. What's difficult is
writing it down.
For every common word, English has a version derived from Celtic, Germanic and early French. Sometimes there are also versions with Latin or Greek descent. If you know any European language (stemming from Indo-European, languages like Hungarian and Finnish are something else entirely) then you'll quickly find words that you can link back to your own language.
Learning all the different words meaning the exact same thing is kind of a chore to really master it, but it's really not a bad language to quickly get started in and have a conversation in for a native speaker of many European languages.
The writing rules are completely arbitrary, though. Written English and spoken English rarely corresponds in any way that makes sense. The easiest example is that you can't tell if "read" is past or present tense, because -ea- has two entirely different pronunciations that are used arbitrarily. There's a good reason for many of these problems (the great vowel shift comes to mind) but that doesn't make the language any more usable in practice. This makes the learning experience a lot more challenging, because without the ability to somewhat reasonably sound out the words you're reading on paper, the learning process takes a lot more time and resources. Languages with easier to grasp spellings just come with better tools.
Every language has is peculiarities and inconsistencies. Even Latin, which at some point in history was standardized as much as possible has exemptions on exemptions to its many rules upon rules. As people speak a language, the language changes, so the older then language, the weirder it becomes. I don't think there are (natural, not invented) languages that are "hard" or "easy" to learn from a general standpoint, because if the language was actually hard and illogical, humans would've changed the way they used the language automatically already. From what I can tell, all languages can be easy to learn as long as your mother tongue is related to them in some way. It could be the grammar structure (like featuring nominative, accusative, etc.) or the way verbs are used, but as long as you can fine some match between your own language and the one you're trying to learn, you'll have an advantage.
I would think English is nearly impossible to teach to a Chinese person, but shouldn't be hard to teach to a French or German person at all. The same goes the other way around: there is much more overlap between Japanese and Chinese then there possibly could be between English and Chinese, so learning those languages becomes terribly complicated for many westerners.
As much as I despised having to learn different languages in middle/high school, I do think that everyone should be taught at least one other language. It'd help the students if that language was something similar to the local language, but for American or English kids it wouldn't matter if that second language was Spanish, German, French or Italian. What matters is the insight you get when you start thinking about language rather than using it. You start to get a feeling for why certain parts of the language are like they are, rather than just being told that this is just how the language works and you should deal with it. For example, explaining the use of "who" and "whom" to someone without an understanding of grammatical cases, which many Indo-European languages still feature today, might be an impossible task even native speakers sometimes struggle with. You could go it about another way and teach Middle English in English-speaking countries to explain these systems, but I don't think anyone would be interested in that.