Want to speak a language that may be geo-politically useful one day? Chinese (either type), Russian, Arabic.
Want a language that may come in handy in the US, especially west of the Mississippi? Spanish.
Many of these can be helpful bridges to other languages. I've heard that Spanish and Arabic have remarkably similar phonetics, and that Spanish speakers pick up Arabic a little more easily than others.
Either French OR Spanish will give you a preliminary boost toward Latin (and vice versa.) If you are headed for the legal or medical field, Latin is good to know.
On the other hand, Russian teaches you the Cyrillic alphabet, which can come in handy as some of the letters crop up in other languages too.
It's never a waste of time to study another language. Never. Heck, I studied Yucatec Maya for two years. All I remember is "Nice to meet you," and "I see you/I don't see you." But it still gave me a perspective on Indo-European languages I didn't have before.
I live in California, so while I've only ever studied French and Maya academically, I'm picking up some Spanish. It's never a waste.
“Chinese (either type), Russian, Arabic”
People never understand the value of those languages. Now, if your kid wants to get plenty of boyfriends or your son girlfriends, have them learn Polish.