I’ve been using Duolingo, a free app, to learn survival Spanish. I think it’s pretty good. Luckily I am immersed in Spanish here in ELP, where folks routinely switch languages mid-sentence.
I also looked at Drops recently; it’s really weird. It’s got an almost-hieroglyphic approach, highly visual and abstracted to the point of inscrutability sometimes. I am playing with it because Duolingo doesn’t do Esperanto.
I am interested in constructed languages like esperanto as they tend to be rational and easier to learn. It’s the best of the bunch but has few speakers.
Maybe an international sign language would do it; it’d need to be symbolic rather than phonetic in order to avoid mother-language bias.