Not talking about comics, rather on books (like actual novels containing chapters consisting of only words). They say that both French and German are “closest” to English but still different languages, however even if an English speaker were going to learn either language, does it mean can they understand written text from books or is that hard even with cognates?
It depends on the genre of the reading material, also affects difficulty regarding vocabulary. Such as: thriller (crime): has terminology, uses expressions and bundled in sentences that are beyond A2 level proficiency while YA (teen) may be written within a beginner lens. This is not the same as reading a sign in French as a English speaker as books have blocks of text.
Even in the reverse: what is the recommended level (from A1-C1+) for a non native who is learning English as a second language to read an entire book (like those written by Lee Child, Agatha Christie) fluently with understanding not only the contents written but infer on the novels concepts and relay reading comprehension akin to a native speaker?
This isn’t really a direct answer to the question, so I apologize for that. I hope it helps your understanding, though…
German and English aren’t mutually intelligible, reading or listening-wise. Neither are French and English. English gets a large portion of its vocabulary from French and Latin, and French also gets a large amount of its vocabulary from Latin. This means that someone who can read English can likely recognize many of the words in written French, but not really enough to understand it.
For German, English shares a lot of vocabulary, as well, but what makes these languages “closely related” is that they are both in the “Germanic Language Family”. They have a recent common linguistic ancestor. Essentially, while English and German may not have a large vocabulary overlap, sentence structure and grammar are similar and historically indicate a recent common language that they both diverged from. An English speaker can possibly understand written German, but it takes a lot of effort and understanding of the changes that occurred in the two languages as they diverged from each other.
Last thing I’ll say is that the vocabulary that English barrowed from French and Latin (which makes up the majority of English vocabulary) isn’t evenly distributed among “genres” or frequency of use. More “fancy” words are likely French or Latin in origin (think science terms and other jargon), while common words are mostly Germanic in origin (think words like “and”, “the”, pronouns, etc.)