"You learnt to read"
Here in the US, the use of "learnt" is considered a very uneducated way of speaking. Sometimes I forget that Duolingo tends to cater to British English.
Duolingo itself doesn't necessarily have a bias - it's the creators of the courses who choose these things. The problem with the Swahili course so far is that not all of the options have been put in, so if you type in "learned", it may not be accepted just because the creators only added "learnt" ... when the course is fixed up, both American and Commonwealth dialect choices should be accepted.