Many of us are surrounded by people whose native language is different from our own. How much more effective we will be as Christ's ambassadors if we speak with them in their mother tongue rather than requiring them to speak ours! Probably the best general advice here is this: study the language you are most likely to use in an informal (non-school) context.
For practical purposes, in North America, the most common non-English language spoken is Latin American Spanish... unless you happen to live in or near eastern Canada, where there is a large population who speak French. Then again, for academic and graduate-level study purposes (Bible scholars are a great example), German is very important.
On the other hand, if your children have a live-or-die desire to study a particular language—for whatever reason—then by all means help them to achieve their goal: let them study that language. Internal motivation will count for much more than whatever long-term practical benefit you may be able to offer.
Many colleges have a foreign language admissions requirement: prospective students must have successfully completed at least two years of foreign language study at the high school level. Even if you have no interest in college-level education, or see no direct and practical application for language study, the truth is: learning a new language is good intellectual exercise. You will certainly walk away from the experience knowing your native language better than you ever did before!
All of these factors together lead us to do what educators down through the centuries have done: we strongly recommend that your students learn a foreign language, beginning by at least 6th grade.