I voted Spanish, French, Mandarin, Arabic, Russian.
Why is there a limit to five? Shouldn't we want our children to be as knowledgeable as possible? Admittedly, with high school, you're limited to only (usually) four years to drill in the necessary knowledge, but in ideal circumstances, for those few classes that high schoolers do have as electives, they'd have virtually unlimited possibilities.
In the ideal, it would be wonderful to be able to choose from all the major languages, but there are fiscal constraints. The school I went to could only afford to offer three languages (Spanish, French, and Latin); I suspect two or three is probably the norm. I think Spanish should be offered by every high school, regardless of location. The second language offered should vary by location; for example, Duluth, GA should obvious teach Korean at the high school level, schools in Arizona and New Mexico near reservations should offer Navajo classes, etc. If the school can afford a third language, I think French is the best choice considering both the number of global speakers, and the historical interaction between the Francophone and Anglophone world which makes French literature more approachable (IMO). If there's room for a fourth class, whichever of Mandarin, Arabic, and Russian that a school can find a qualified teacher for would be good; all of those are spoken by hundreds of millions of people in.