I've been learning Mandarin, and after searching for something like Duolingo found Chinese Skill [一]. It's not as good as Duolingo, but does include a way to teach characters. It would be great if it could have a drawing interface like Pleco [二] which can reliably recognise most characters I draw, with my terrible handwriting.

They do teach English to speakers of Chinese, Mandarin and Hindi.

[一] https://play.google.com/store/apps/details?id=com.chineseski...

[二] https://play.google.com/store/apps/details?id=com.pleco.chin...