Check the “wifi” symbol to know if the app needs an Internet connection to work!
For most volunteering websites such as Workaway or HelpX, there is no app available (yet). Only Woofing Australia and Worldpackers (free download, registration $49 / year / person to contact hosts) stand out with an application available under Androïd. However, as we are not registered on these sites, we can’t give you our opinion on it…
We just call for developers to fix things!
However, if you want to improve on your side, you can find free courses on Youtube by tiping “Michel Thomas” or “Assimil”. For having tried “Michel Thomas” courses of basic Russian, I enjoyed its interactive side with breaks to be able to assimilate, repeat the new words learnt or build simple sentences.
NOTE: There are other applications to learn a language, but paying: it is Babbel, Memrise or Busuu.