If someone is unfamiliar with Japanese and Mandarin, they’re not going to be able to readily distinguish the two in writing. They don’t know, for example, that Japanese intersperses the Kanji with Kana, while Mandarin uses the hanzi logograms exclusively. But Chinese characters are very distinct from Latin or other Western writing systems, so if they see a sample containing Hanzi/Kanji, they’ll jump to whatever they’re familiar with.

I imagine a Chinese person with little to no exposure to Western writing would easily mistake French and English because they share so many words, even though they’re not closely related (Romance vs Germanic).