Speaking English and Swedish fluently, I'd say Swedish (and Danish/Norwegian) is simpler grammatically, as it just has the one conjugation per tense regardless of the personal pronoun: I are, you are, we are (är). I were, she were, you were, we were (var). This is why you'll hear Swedes/Scandinavians make typical mistakes like "we was".
As an aside, it also seems to me like Swedish culture is more monolithic/conformist and contains a small set of really common idioms and phrases that get you a long way towards being conversational. It shares that aspect with Dutch as spoken in the Netherlands, as opposed to Dutch in Belgium.
> English became the modern lingua franca
As a second language speaker, I can't help but think poor English is the lingua franca. Not to mention lots of couleur locale in the form of accents.
The deceptive thing, though, is compound tenses and phrasal verbs; in both cases you express a meaning with multiple words that couldn't have been predicted from the meanings of the constituent words alone. These are both pervasive in English and both very tricky for learners, but they often don't feel like "grammar" because they're not inflectional. (Well, phrasal verbs are more like lexicon.)
I eat breakfast
I am eating breakfast
I ate breakfast
I was eating breakfast
I have eaten breakfast
I have been eating breakfast
I had eaten breakfast
I had been eating breakfast
I would eat breakfast
I would be eating breakfast
I would have eaten breakfast
I would have been eating breakfast
I will eat breakfast
I am going to eat breakfast
I will be eating breakfast
I am going to be eating breakfast
I will have eaten breakfast
I am going to have eaten breakfast
I can't think of another European language that would idiomatically use (or maybe even require) quite this range of constructions.
All languages have the sort of contradictions that are pointing out there, and English isn't especially bad (in fact, most of the examples there are directly inherited from German). Most of the difficult things in English come from it being a Germanic language with a bunch of French bolted onto it. That's why its spelling is so atrocious and there are a lot of duplicated words with very similar meanings in common use.
But, blessedly, English does have the simplest grammar of probably any European language. (I've occasionally heard that Swedish is the closest in simplicity, also being a Germanic language that's dropped most of the complexity, but isn't quite as inundated with loanwords.) Aside from a couple of concepts (the gerund, specifically, which isn't common in some European language families), most grammatical structures in English have direct equivalents in all other European languages.
However, going the other way is difficult: most European languages have formal and informal voices, gendered nouns, some have irregular plurals (German, notably), subtler and more verb tenses (Romance languages), and complex case structures (German and even worse, Slavic languages). If you're a native English speaker that doesn't know what cases are, for example the word for "the" in German changes based on which of the three genders the noun is and how it's used. It could be "die", "der", "das", "den", "dem" or "des", and the endings of words that need to be in agreement with it change as well.
The main reason English became the modern lingua franca is the British Empire, but it does help that it's also a very easy language.