Your first language is hard because you're learning a ton of things all at once, only one of which is the language itself.

Your second language is hard because it's not the first, even though it's probably pretty similar. But the differences will catch you out. A second language challenges the mental models you built up with the first language -- you have to tear some of your understanding down and come up with a more generalized perspective.

Languages similar to the first two become easier to pick up after this point, because you're mostly looking for a handful of differences, not a total paradigm shift. But languages that are sufficiently different, like C and Haskell, require a lot more of the ground-up type of work you thought you'd left behind by now.

It gets easier and easier as you cover more of the conceptual landscape. But I'd be willing to bet that many developers haven't had to journey far from home.

I’d agree that similar languages are easier to learn, but once you’ve learned how to learn a programming language, new ones are easier, even if they’re dissimilar from your prior languages.

For the record, my professional work includes languages as dissimilar as PHP, Java, and Common Lisp, while my hobby hour work includes Rust and Haskell. That’s a pretty wide range of the language space that exists today, IMO.

My language gamut is similar to yours -- I don't disagree. I usually find it straightforward to pick up something new too. But there are still languages I know are going to take a lot more time for me to grok, like Coq. (I tried Coq a few years ago, and even though I already knew a fair bit of Haskell, the differences were enough to completely throw me off.)

I agree with you. I think there are some problems with the study. For example "Does cross language interference occur?" Not only does it occur, but that's literally how you learn a new language. You make assumptions based on the languages you know. You write code. You notice that it doesn't work the way you expect. You try to understand what is different. It's by far the fastest way to learn a language rather than a problem.

The learning strategies of successful language learners show this: Learning on their own, Just in time learning, Relating new language to old langue. I would say that the "Language interferences" are just a product of the learning method. Putting yourself in the deep end and trying to swim always results in a little water in you lungs ;-)

The fact that it is challenging does not imply that it isn't desired. In fact there is a whole branch of skill acquisition theory called "desirable difficulties". While I think their theories of what would make the process better are interesting, I don't actually agree with their conclusion - that we should proceed as if their theories are correct.