I was taught that teenagers acquiring English as their second language are best off when they can continue learning domain specific concepts in their native language while waiting for their English to mature enough to transfer this knowledge over. Otherwise, they gain conversational English fluency but miss out on their first, best chance to learn foundational abstract concepts in, say, Science, Math, or Social Studies, leaving them without the ability to talk or even think about these subjects in any language.

Do you remember the source for this? Because what you described here was very fashionable in my country, at least a few years ago -- having schools where students not only learn English (as a second language), but also learn all subjects in English, so it deepens their language skills.

As any programmer will tell you, leveling skills in one language will have knock-on effects on your abilities in other languages, whether they are similar or not

Confirmed. On the other hand, there is also something that I call "writing C++ code in Java", that is: forcing the idioms of one language to another, even when the other language provides specialized tools to deal with the situation.

(Specifically: Someone with a history of C++ will prefer to use abstract class hierarchies, complain about the lack of multiple inheritance, and consider interfaces to be merely an inferior form of classes, when writing a code in Java. While the natural way of writing Java code would be to construct interface hierarchies, and treat classes as mere implementations of them. I am writing this here because it took me a few years to grok this.)