Old English was a Germanic language primarily. After 1066, the court language of England became French and remained so for centuries. It was in this period that the Middle English transformation happened.
The fascinating thing about this is that the language became much more regular in that period and, more importantly, it dropped a lot of what I as a native English-speaker at least consider pointless grammatical cruft.
For example: Old English had 3 noun genders (male, female, neuter; this being the norm for European and Semitic languages). Middle English lost that (other than male and female for people). Old English had 5 cases. By comparison, modern German has 4, Latin has 6, some Eastern European languages have more. The concept of case has almost completely disappeared from English (pronouns and the Saxon genitive notwithstanding).
It's fascinating because it seems what keeps languages unchanging is a ruling class. It's a bit like the philosophical view of grammar as being descriptive (my view) vs prescriptive. It's almost a model for conservatism being the resistance to change.
As a native English speaker I've found it difficult to learn other languages not just because of the ubiquity of English but because other languages have concepts that English just doesn't, like in German where adjectives and pronouns have to agree by case, number gender and article of a noun (eg "the" vs der/die/das/den/dem/denen/der/des). That structure just seems like such pointless cognitive load, at least to learn. I'm sure it's zero cost if you grew up with it.
But it does seem like it makes English easier to learn. Obviously there are some complexities in English (eg adjective order and the tenses).
And before anyone mentions Asian languages for simple grammar let me just point out: no clear word separators and the writing system in general (although this varies too).