I'm not sure if it's used in the UK. I was speaking of the southern U.S. It's entirely regional, but it's widely used here. Your first comment gave the impression that no one uses it. I do agree that it's an odd choice for Duolingo to use. At least reckon is an actual word. I'll be more worried when I see "fixing to."
Yes, I just thought that the word was so regionally associated that it was unusual and unexpected for DUO to use it as an option for "I think." i would be equally surprised if they used verbs like glean or tote. I don't reckon that I was trying to make fun of people who use reckon either.:)
Spanish uses the definite article (where English doesn't) when you're making a generalisation. I.e. you're making a statement that you claim is true for every instance of the subject. In this case you say "Sports are fun", so you're saying: whenever there's a sport, it's fun.