"I met him for the first time in Poland."
Translation:Mi unuafoje renkontis lin en Pollando.
Name of Poland is in the category of names based on the nationality, so we have a noun “pol·o” (Polish) and we could say “Pol·uj·o” (as it is with words from this category). However, there are few words from this category (six, to be precise), with which it's recommended to attach “land-” (country) instead of normal “-uj-”. So we combine “pol-” + “land-” + “-o” and get “Pol·land·o”. :)
Thank you very much that this time at least you didn't go with the ambiguous, not accepted by the Academy pseudo-infix “-i-” and didn't make my country sound like a disease.
However, I still think that for the purpose of teaching Esperanto to beginners one should present the most correct, official and widely accepted version of the language, so please change country names in other questions to “Francujo”, “Rusujo”, “Italujo” &c. Thus they will be unambiguous, understandable and correct at the same time.
Regardless of whether you think it's a good or a bad thing, the -io form of countries is very commonly used nowadays. I think it's good for Duolingo to take a descriptive, not a prescriptive approach—even when dealing with a conlang like Esperanto. You want to be able to speak and understand the language as it is used, after all—that's the goal of a language course like the one here on Duolingo.
However, it's fine to put a note in the tips and notes about different opinions and controversy about it, of course.
„Widely accepted“? No!.. :) It's wonderful that the course teaches Esperanto as it is, not as it was before the October Revolution :) Languages do change, and Zamenhof wrote himself that the community should decide how the language will evolve. It evolved to Rusio, Italio, Pollando.