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
I'm living in America. I just try to use the translation given when we get the word, then read comments to see if another word is also accepted. I think it's fascinating to learn of regional differences. In my part of America, I'd only use "trousers" for briogais" if I wanted to sound snobby or out of touch. I'd use pants, jeans, or possibly slacks. Even the nice men's stores sell suit pants, not suit trousers.