What's the hardest part of learning a new language?

Vocab is the easiest, you get a basic grasp of the grammar, read books/watch tv, write every word you don't know on a notecard and learn it. Even easier if you take classes where they hand you an actual list of words to learn.

Grammar can be hard, I've been learning Spanish for about 5 years now and I still on occasion make stupid mistakes like mixing genders or using the wrong verb tense.

Pronunciation heavily depends on the level you want to take it. If you just want to be understood with an accent, it's easy. If you aim to sound like a native, you are on a long hard journey with your language.

Anti-translation comes with practice, when you've heard a word or phrase said enough times, you start just seeing it as less foreign and just an alternate word for the same thing.

I would have to say other though, more specifically, listening. Hearing a language coming from fluent speakers is rather hard to decipher because

A. They move faster than you can keep up with (related to the translation option)

B. They subconsciously eliminate small sounds and jumble their words together making, for example, 'Hey, how are you' sound more like 'Hehowerya'. It's much less clear than a lot of slowly spoken things you hear in a classroom. This means that you might hear something as a single word when in reality there were 2 or 3 present.

C. One word you don't know in a sentence can really throw
your entire comprehension off.

I know 3 different languages & the hardest thing for me is attempting to not mix them up. I'll say a sentence in one language and accidentally throw another word from another language in there. LOL

:)

I think listening is very difficult when you're trying to understand the language coming from fluent speakers. If you're in the language's native country and surrounded by the language every it usually takes 2-4 months to be able to understand everything. And even then you don't know all the vocabulary of course. So I think vocab is the easiest because if you know everything else you can slip in new words you've learnt.
But I think pronunciation is very important as well. The slightest difference in sound can mean something completely different so perfecting that would come with lots of practice, and the best way to do that is go to a country where the language is spoken.
For every individual it's different. To really master pronunciation you need to be able to manipulate and imitate each sound perfectly. Some people have real trouble with this and that's how we get strong accents and misunderstandings.

If you don't practice. When I was taking Japanese I the hardest thing was trying to get the use of everything without actually having someone to practice with at home.

learning the language.

The hardest thing about language learning is to master an idiomatic level - to know the way to express certain idea or question in a native form and not merely literally translate it. You can be proficient in grammar but still unable to say simplest things. Language is more about exceptions than rules.

That damn grammar. I took French for three years and had to stop myself from taking it more than that because I struggled so much on quizzes. I would have liked to continue, but my grades got worse over time.

Everything including being able to remember them:P

I was once learning Spanish and then I completely forgot most of what I learned.

The only language I really know is English, but I'm starting to forget it too:(

It largely depends on the use you are planning to do of that language and the use you did of your own.
For somebody with a speech pet peeve the most difficult part might be pronunciation. The whole fonetic part. Spelling the words out right. I didn't actually learn the language, I simply absorbed speech patterns and styles from music/movies/books I mix up randomly.
This "actually" repeating thing, at the beginning of most sentences, for example, got stuck in my mind after watching a particular american dad episode involving Jeff, Stan's daughter stony boyfriend. The "for example" got stuck in my mouth after watching Hermione saying it in Harry Potter. I started using the "whole thing" expression after watching a documentary about the Manson family. I was like: Right on! All through the documentary. :) I learned more from movies and tvshows than from school. The hardest part was.. retrieving the files. Not really.
Sprechen sie deutsch?

Just everything there is to know about it. I've taken four semesters of French and I still can only say, "Hi, my name is ________," and that's it. Although I managed to remember the colors.

Vocabulary for sure.