Translation:Do you reckon that that berry is poisonous?
Sure, my dad used it all the time. While I would use "think" in this context, "reckon" is perfectly acceptable, even if it sounds a bit "folksy" to my ear.
Quora had a several good responses about this. While Duolingo won't let me post directly to it, search Quora for "Do Americans ever use the word 'reckon' or is it strictly a British word?"
I no longer have the book. It was a book about the regional varieties of American English. In standard English, we say, "I suppose..." In some places, Americans said, "I guess..." In others, they said, "I reckon..."
It is similar to "soda" and "pop". In some parts of America, they say "soda". In others, they say "pop". In an episode of "Monk", Mr Monk deduced that two men were not brothers because one said "soda" and one called it "pop".
The default translation should be "suppose" or "think."
"Reckon" with this meaning is dialectal, and you would probably not see it in English kirjakieli unless the intent is to sound "folksy." In standard English it means "calculate," either in terms of mathematics or analysis of data, but is nowadays mostly limited to the gerund, as in the phrase "By my reckoning" = "According to my analysis."
Just my input as a native speaker: "reckon" sounds folksy, dialectal, even unlettered to me. I guess it is a standard English word in some parts of the English speaking world, but nobody I know would use it unless trying to be folksy. I would use "suppose" or "think" instead.
Does the Finnish word carry a similar connotation? If not, it should probably be translated differently.
I'm an Australian-English speaker and I've discussed "luulla" with my Finnish partner. My understanding is that it's extremely close to the connotations of "reckon" in my dielect i.e. not necessarily informal but suggesting of a more specific amount of thought. More like a hypothesis than either a hunch or a fully fledged thesis.
I reckon "think" is probably the best choice in American-English.
Finnish has so many verbs with different meanings that all translate into "think". We have to keep them apart somehow or we won't be able to teach the difference due to the limitations of the platform. While the primary translation is "reckon", "think" and "suppose" are also accepted. :)