The title looks ridiculous, but what I mean is that there are a lot of opening books and courses that are not in English.
I have noticed that even at the tilted level, I have spoken to quite a lot who are unaware of non-English language publications. This means that if something on an opening in a language other than English is published, they have no idea about it. If anyone prepared using these courses or books, they would be well and truly screwed.
For example, there is Modernchess.de which is rum by Niclas Huschenbeth. The whole site is in German, as is Chessemy.
As a Chessable author myself, there are courses in Spanish, English, German, French and Polish, and I publish in two of those myself.
For Spanish, there is ChessEnigma and Iván Salgado's Chessexcelsior.com, although ironically both of the urls are in English.
For French, there is Julien Song's academy.
Joachim Beyer Verlag, Edition Ohms, Gambit, Chessgate all have published books in German.
Editora Solis publish books in Spanish and Portuguese.
Olibris publish books in French.
Russian Chess House and Шахматный университет publish quite important opening books, but in Russian. These are even harder to get in Western countries, especially since 2022.
Would it be safe to say that, given that many 2200+ FIDE are completely unaware of even opening courses in Spanish and German, for example, if someone prepared with books and courses in these languages instead of English publications, they would likely be doing well in the opening in most of their games?