> The hardest part about learning a language is knowing what to learn.
I disagree with this. The hardest part about learning a language is finding content and resources in the target language that are both engaging and comprehensible at your current level. It's hard to find books, shows, or conversation partners, all the way through that critical awkward stage from beginner to vaguely proficient.
The parts of the language you need to learn will reveal themselves to you through your interaction with it. And then, either your brain will figure it out intuitively, or you'll be able to look things up if they remain a mystery.
Perhaps being equipped with a base set of phrases will increase your range in the very beginning, but that does involve offloading a lot of comprehension work to your brain's CPU, rather than letting it be handled by your brain's in-built language processor GPU. I'm suspicious about any such strategy for language learning.
Context clues that accompany the language input are a much more powerful way to make language comprehensible, compared to memorizing phrases and adding strain to your already busy brain.