Learning certain languages as “prerequisites” for learning other languages.
FFS, this isn’t school. No one not high on laudanum would suggest you learn Latin and Greek (or even Italian and French) and Frisian before learning English so that learning English would be easier and your English “purer”. But somehow there’s an idea in the stupid section of language-learning circles that one “should” learn language X before learning language Y.
Languages aren’t lubricant for each other. One doesn’t make it so that the other slides in easier. Not in the way you’re thinking. The second language helps you learn the third language not for linguistic reasons but for psychological ones: you’re full of confidence 1 from the knowledge that you’ve handled another language before; you not only know that it’s doable but that you can do it. 2
If anything, linguistically, languages interfere with each other. If they’re too far apart, there’s no reinforcement. If they’re too close together, there’s confusion, false friends and the pernicious, unconscious tendency to mix and overlap (in fact, even if they’re far apart, there’s still that tendency). Either way, you need to train yourself to keep them separate while also keeping each one healthy — logistically, there are time, media and other resource allocation issues to resolve.
So, yeah, don’t learn one language so you can learn another. That’s like forcing yourself to eat crap food so you can better “appreciate” good food: you just end up developing a taste for crap food. Again, no one not high on anachronistic narcotics would suggest you listen to all jazz and only jazz before you listen to any hip hop, so that you’ll “get it”. It may be an interesting academic exercise, but so is electrocuting gay people in the bollocks to de-gay them. In practical terms, it’s the dumbest idea ever. Not good dumb. Dumb dumb. Remarkably dumb. 1980s fashion dumb. The dumbest idea ever, dumb.