So I’ve seen a comment about learning Spanish making you get a little grip on Portugeese and Italian, my own language helps understand our neighbors.
I wonder, how to abuse that system for the most efficient pick of 3 or 4 languages to rule them all? Let the bar be just reading, text as simple as social media posts.
Again, not people (or we can just put this link, but languages treated as autonomous entities by science.
That raises an important point: the Chinese language(s) are actually completely unrelated to Japanese, but the writing systems are related—and their partly-semantic nature lets readers recognize some isolated written words (with no indication of pronunciation or syntax).
Does that meet OP’s criteria, since they said they were mostly interested in reading?
If you learned Mandarin, but learned to read Pinyin instead of kanji, the mutual intelligibility of other languages would be totally different.
To add further context–I’d like to emphasize that an understanding of written Chinese would help with Kanji, but like you said, to a limited extent. When reading Kanji, there are cases where you’d have to be cognizant of Onyomi and Kunyomi (Basically pronunciations rooted in Chinese vs. Japanese). Not as important if you are strictly “reading”, I suppose. However, this would also not provide insight when reading Hiragana nor Katakana, how particles are used, rules for conjugation (polite vs. casual, past vs. non-past tense, etc.), further reducing mutual intelligibility. In some cases, Chinese characters may be visually identical to Japanese Kanji, yet have different meanings or applications. Traditional Chinese vs. Simplified Chinese is also a whole other topic.
Examples where there is some similarity:
JP: 走る
EN: Run (verb)
CN: 走路
EN: Walk (verb)
Matching characters, unrelated meaning and application:
JP: 勉強
EN: Study (noun)
CN: 勉強
EN: Reluctantly (adverb)
Furthermore, Chinese uses Subject-Verb-Object (SVO) word order, whereas Japanese uses Subject-Object-Verb (SOV) word order. Japanese also regularly uses subject omission, so it’s important to consider these things if you’re moving from one language to the other. Missing an understanding of these differences could lead to pretty different interpretations of a sentence.
That being said, having a background in Chinese would be more beneficial when picking up Japanese than the other way around, IMO.