Bring offline version. PLEASE!!
There used to be some offline kind of possibility... some lessons were saved. Not anymore!! Please please, bring some sort of offline version, even if it's just a basic set of exercises for daily practice. Keeping this daily routine of language learning / practice is, in my opinion, key to learning faster and getting used to the challenge of it.
There is an offline version of the app (but not the website): https://support.duolingo.com/hc/en-us/articles/115004068043-What-is-Duolingo-Plus-