Here's the story of my weirdest language study situation:

I once took French taught in French; basic German taught in French; and absolute beginner Arabic taught in French.

At the same time.

I refused to translate classes into English.

Absolutely rewired my brain.

I was at school in France. Had semi-functional French, had previously taken beginner German, was starting Arabic from scratch.

I had F-G and F-A dictionaries. For French homework, looked up words I didn't know in a children's French-French dictionary.

Let's say I looked up "Die Taube" in my F-G dictionary. I'd get back "colombe."

Then I'd look up "colombe" in my children's dictionary and get something like: "Oiseau de la famille des columbidés vivant en plaine et dans les villes."

If I didn't know a word in the definition, I'd look that word up in turn in my French children's dictionary, too. And so on. Until I worked it out.

I read classical French literature this way for my lit class. Including Les Liaisons dangereuses by Choderlos de Laclos (1782). https://twitter.com/ShaulaEvans/status/1314347503911792640

I read novels that I took out of the library this way, too. That's when I read L'écume des jours by Boris Vian.

(I might have read it for school? Fairly sure it was among the books I read on my own. It's been a while and I may be misremembering.)

I kept notebooks of the words I looked up. I'd write the definitions out in French ink cartridge pen on that weird multi-lined French notebook paper (I probably still have some in a box at my mom's somewhere).

Years later I also studied (Brazilian) Portuguese in Japan, taught in Japanese, by a lovely Japanese-Brazilian woman. That was also a mind-melting experience but in a different way -- smaller scale, lower intensity.

Thinking back to those classes with language study on my mind tonight.

If you have any level of proficiency in an acquired language, you might give an experiment like this a try.

It isn't necessarily efficient but it's really interesting.

Oh! The end result is I have pockets of second languages in my brain that are completely firewalled from English. I know words in other languages that I have no idea how to translate into my first language.

And that is really, really cool.