"We speak Irish."
Aw I hadn't yet learned that "Tá Gaeilge againn" also means "We speak Irish," but now I'm curious of its literal translation. :)
If you were just to say " We speak Irish" and not quantify it, (with, say "in class"), and to not mean habitually, Tá Gaeilge againn is the native way of doing so... The other is normally used when you quantify it. (e.g. Tá Gaeilge agam is the way most natives would say "I speak Irish" without quantifying it). It literally translates as "I have Irish".
You mention two different statements in Irish, both with a common translation in English.
The contributor most likely started with the Irish sentence and provided this perfectly reasonable translation in English. The program has, however, introduced ambiguity by requesting the original Irish sentence, but there are now two possibilities, although the original intention was to teach only one.
The bí ... ag ... form is a statement of language proficiency i.e. that one is able speak Irish. Literally, to have a language, or even more literally, be ... at ...
The use of labhair in the present tense describes language usage i.e. that one speaks Irish habitually in an unspecified context.
Given the context-free nature of Duo, both could be considered correct here.
gaeltacht is generally used for an irish speaking area, there are lots of towns particularly in the west (connaught) that only speak irish and those are called gaeltachts. there are also gaeltacht schools where everything is taught in irish and during the summer there are camps that are just called gaeltachts where you have to speak irish or else your kicked out. theres one called lurgan and at the end of the year they film a music video of a pop song where they get everyone to preform it in irish, heres a link to 2016s- https://youtu.be/G1SlZLWjhvg
I am pretty sure I will never remember whether is is "ai" or "ia". I get these wrong over and over again. What is the difference in sound? Is there a difference. I would so much rather sound things out than try to memorize.
Have you learnt the "broad with broad, slender with slender" rule yet? If not, look that up and start with that. The best way I found to learn the spellings was to just write them out on paper over and over. I wouldn't rely on duo as a sole resource.