Most foreign language programs will not be continued to the point where all, or even most, students will be functionally bilingual. That is sad because early bilingualism makes permanent structural improvements to the brain.
These aid concentration and problem solving and fend off dementia later.
For best results, the learning needs to be both thorough and early.
Esperanto, as the quickest and easiest language, will deliver the benefits to most children soonest. See more here.

As well as upgrading the brain's machinery, Esperanto gives it a good workout:
Literacy skills, problem solving skills and thinking skills are developed in learning any language.
However, students learning Esperanto spend more time practising higher order thinking skills such as application of understandings to creative tasks, synthesis and analysis of texts and evaluation processes. This is because less time is needed for recognition and recall in
Esperanto, because it lacks the idiosyncratic variations in spelling, grammar and pronunciation found in national languages.