Again, the difficulty of language acquisition is determined by how similar the new language you are trying to learn is to the languages you already speak. There isn't any kind of objective measurement of language acquisition difficulty.

For whatever reason people are often very resistant to this fact, but just speaking several languages fluently doesn't mean you actually know shit about linguistics, and this is a very well-established concept.

So a native Mandarin speaker would probably find English and German equally difficult to acquire because they are both so unlike their own language.

Fun fact about English; Old English was even more grammatically complex than modern German but it got stripped down over the years and now accomplishes through syntax much of what it used to do with grammar.

There are also a couple of very odd qualities to English that may have come from the Celtic languages, but the idea is still pretty controversial.