People are just repeating this advice about making your own decks, and it's based in nothing but having had it repeated to them. Spaced repetition is boiling in pseudoscience and ancient studies that don't say much other than that there's a forgetting curve.
Most people are just parroting stuff they read on the Supermemo wiki (or somebody read off the Supermemo wiki and repeated to them like they came up with it), and all of that is just thoughts off the top of one guy's head. His innovation is that he wrote a program to do Leitner boxes before he had ever heard of a Leitner box, but people treat every word like gospel.
The only five things I can say for language learning is to go really hard on systems in a new language that are completely unknown to you (like Romance conjugations for an English speaker); only drill sentences, not individual words; always say your Anki answers out loud, and read out loud as much as you can; comic books have pictures, too; and once you get comfortable in an L2->L2 dictionary, you're a more comfortable reader than a lot of natives.
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* Random Anki decks for a few European languages: https://sookocheff.com/post/language/cloze-deletions/
(Edit: the lovely thing about 10K algorithmically generated clozes is that they're utterly disposable, unlike cards that you make yourself. If one is a leech, forget about it. You'll see another one just like it when you get to the point that it won't be a leech for you.)
* Instructions on how to generate your own in other languages, for developers: https://sookocheff.com/post/language/bulk-generating-cloze-d...
(You could probably point out the above URL to an LLM and it would generate the code for you.)
* Anki to learn Romance conjugations first: https://www.asiteaboutnothing.net/w_ultimate_spanish_conjuga...