I have studied other languages. I've got a talent for it. I'm just going to be honest with you, which is better than some of unrealistic answers you've been given so far.
The problem with Chinese is the tones. Depending on your genetic material, as an adult you may find it very difficult to come to grips with them. Or it could be easy for you. But I can promise you that for every person for whom it is easy, there are tons of native English speakers who will never be able to deal with it successfully. The grammar in Chinese is pretty easy for the most part, which is good, but the tones are the killer. I am always amazed at how people suggest learning Mandarin or Cantonese without any regard to the difficulty that speakers of non-tonal languages will have. And you need to understand that as an adult unless you want to devote the next decades of your life to constant work at it, you will never learn Chinese characters. Yes, you could learn pinyin but that's not really all that practical honestly. So for all practical purposes you will be illiterate in Chinese, even if you learn to speak it well. Yes, you can use programs to translate your pinyin into the characters and vice-versa, but how practical is that on the streets of Beijing?
Yes, if you want to engage in questionable activities then Russian would be a good choice, but I can tell you that most native English speakers fail at their attempts to learn it. I'm one of the exceptions. Russian grammar is quite complex. It is an inflected language and that's the complexity. What this means to people not familiar with linguistic terms is that Russian nouns and adjectives change their spelling depending on how they are used in a sentence. Russian adjectives have up to 24 forms - 6 cases X 4 forms per case (singular masculine, singular feminine, singular neuter, plural). The good news is that some of the forms overlap so in reality there are usually "only" 19 or so forms to learn. Ha ha. Nouns have singular and plural forms to learn. Given how in the USA most English grammar instruction is over forever in public schools after 8th grade, you really have no idea how challenging it is for someone who doesn't even know what an indirect object is in English to try to understand something like the dative or genitive case. Without a proper understanding of the cases in Russian and memorization of the various forms of nouns and adjectives under them, you'll never make any progress at learning it. Outside of the ex-USSR it's generally pretty useless. I get some kicks out the "wow" factor of being able to impress people that I can speak it and I've done some traveling in the ex-USSR where I used it every day, but in the IT world it's been almost useless. Then again, I'm not a leet haxor. I can tell you that learning Cyrillic is very easy and that will absolutely not be the problem in learning Russian, but the grammar will separate the men from the boys. If you can believe this, from a grammatical standpoint most of the Slavic based languages are actually harder to learn than Russian, with Bulgarian/Macedonian being an exception.
English is really the most useful language to know. If I had to recommend another language, Spanish is generally the easiest one for English speakers to learn. Portuguese is not bad either. French would be next, followed by Italian and German and then pretty much everything else. The further English speakers get from Western Europe in the languages they want to learn, the more difficult it will be. I've found that the older you are, the harder you have to work at learning another language and most adults aren't willing to do the hard work necessary to succeed. Unless you are some language learning genius (unlikely), you will need to do about an hour a day, 5 days a week for about a year to achieve any kind of reasonable proficiency. And it's like climbing a hill. Once you get to the top, it's much easier to get down, but many give up on the way to the top because progress is so slow that they decide it's not for them. It could be many months of hard and constant work before you start to make any real progress. Most people give up before they get over the top and it starts to get easier. Unless you are going to work for a foreign company that uses the language you learn or in a country that speaks it, there's not a lot to be gained by learning it. I do some foreign travel so being able to speak more than English is useful to me.