Wouldn't it be a bit easier... a lot easier to stick to languages that fall within the same family or at least have something, ANYTHING, in common? I don't say this to deter you but the languages you've picked are all very difficult and Russian has nothing even remotely in common with the other two, afaik. I don't know anything about Japanese or Mandarin but, after looking through Wikipedia, at least Japanese uses a lot of loan words from Chinese which, I imagine, will make it a somewhat easier to understand the two when you start to dig into etymology for a broader understanding. The point is, I think you should scratch either Russian or Japanese AND Mandarin from your list. Learning both Mandarin and Japanese will take a long time (lifetime) but at least there's something in common between the two (loanwords, beyond that I have no idea if there's anything in common at all) even if it's small... Unfortunately I imagine (because I genuinely have no idea) that the similarities between Japanese and Chinese are about as close as Russian is to English so... good luck! but at least there's SOME relationship, something that might reinforce your knowledge of one language with the other, no matter how small.
If you said to me that you were wanting to learn 5 different Romance languages then I'd say go for it because that's absolutely a doable thing. At that point the hardest thing you're going to face is having an acceptable accent in all 5. I don't really speak any of those languages but you could show me the title of some scientific paper written in Italian and I could probably decipher most of it just because of how ingrained Latin is in English and the sciences... in that sense of learning, you're just taking some existing linguistic foundation, strengthening its roots and creating new branches instead of trying to plant some entirely new and foreign tree.
English? Go Dutch (easy and simple grammar/structure with many English cognates), Nordic languages (Swedish or Norwegian) or German (the "same" as Dutch but the structure is way more complex)
English is such a strange bastardized language. I was watching some videos on Dutch grammar and the lady said werkwoorden to mean verbs and it really just angered me that we don't have or use so many compound words like werkwoorden when teaching grammar in English. How much easier would it be for a child to understand workwords over verbs? Simple words with a prefix and suffix that already have some real associations inside a child's mind.