As someone who's had to (or maybe wanted to) learn both english and german, I'd say german has a steeper initial learning curve, but it becomes easier later, whereas english can be easy to get going, but it's difficult to master (obviously the spelling, but also the more advanced use of tenses etc).
Regarding Twain's complaints, the biggest lesson I learned from learning german at an older age was: languages are nonsensical, illogical, and the sooner you stop trying to make sense of it all, the faster you will advance. Children do not learn languages by being analytical and by comparing to what they already know, they learn by just accepting it as it is. This leads to a funny phenomenon that foreign speakers know the actual rules of a language better than most native speakers, simply because native speakers know things are as they are, but not why. Whether adults are capable of this is a different question, but trying to find logic in a language will only lead to frustration.
Regarding Twain's complaints, the biggest lesson I learned from learning german at an older age was: languages are nonsensical, illogical, and the sooner you stop trying to make sense of it all, the faster you will advance. Children do not learn languages by being analytical and by comparing to what they already know, they learn by just accepting it as it is. This leads to a funny phenomenon that foreign speakers know the actual rules of a language better than most native speakers, simply because native speakers know things are as they are, but not why. Whether adults are capable of this is a different question, but trying to find logic in a language will only lead to frustration.