I speak two fluently. English and Russian. I want to learn another language. What's your suggestion?

Update: Recommend languages for my 7years child, Thanks in advance.
Update 2: Guys, Thanks for your suggestion. As many of you recommend Mandarin Chinese, I google it with Chrome. And find Lingo Bus is good at kids chinese teaching. I booked a demo class just now. https://www.lingobus.com/
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