Duolingo is an American website, and learners come from all over the world. The standard is to use American English. A wee excerpt from the Duolingo Community Guidelines:
Embrace and share regional language differences
A language can have many words, accents and ways to say the same thing. We think that’s one of the wonders of languages. Approach these conversations with an open mind and attitude.
If you would like to take a look at the Community Guidelines, they can be found here: https://www.duolingo.com/guidelines