Languages you use to learn OTHER languages in

Hi, I'm a native speaker of English but amateur polyglot and linguist. Of course I'm used to all that comes with being a speaker of English, including that language materials for most languages are often only available via English. Some notable exceptions include if you, say, wanted to learn Nahuatl it's very helpful to speak Spanish so you can access the larger body of Spanish-language materials made for Nahuatl, etc.

Living in a Slavic country and learning other Slavic languages, I actually prefer to buy a book for learning Slovak, for example, that's in Polish (a language I already learnt some of). It's because the grammar parallels are of course super strong and skip the stuff I'm used to. Compared to an explanation targeted for English speakers learning their very first Slavic language.

So I have this personal opinion that I like using materials written in (language I already know a bit of) to learn a related language I don't know yet. Using Spanish written materials for Italian for example, or Polish for Slovak.

I know some people feel differently and get used to just using ONE academic language they associate with language learning, to process all. What are y'all's opinions or feelings on languages that are useful as a medium for learning yet another? Reasons for and against various languages? Under what circumstances...what's your native language and what languages are your final goal?

Ps other fun combos I discovered were the need to use French to learn Breton (bad idea, only makes Breton even harder) and for price reasons, to tap into buying materials for Icelandic written in German (actually sensible since they have plenty of grammar and vocab parallels).