I highly recommend monolinguals learn Esperanto first. Duolingo's finally got a course open for it.
https://en.wikipedia.org/wiki/Propaedeutic_value_of_Esperanto
>Several studies […] have concluded that one year of Esperanto in school, which produces an ability equivalent to what the average pupil reaches with European national languages after six to seven years of study, improves the ability of the pupil to learn a target language when compared to pupils who spent the entire time learning the target language. In other words, studying Esperanto for one year and then, say, French for three results in greater proficiency in French than studying French for four years. This effect was first described […] in 1908.