Wiktionary gives the plural of 'lacha' as 'lachain'; if this hadn't been Irish, I would just have reported it as a mistake, but I am sure that there some subtlety here, that I'd love to have explained to me!
The plural of Lacha is Lachain/Lachainí. Lachan is the genitive, so this is a mistake.
This one just popped up in the Strengthen Skills I'm currently in the middle of, so I'll report and quote the above correction now.
ETA: It's now reported.
Sorry if it's a dumb question, but why can't you say "ta lachan siad" as an alternative?
It's explained here: https://www.duolingo.com/skill/ga/Basics-1
Basically, if the concept you're getting across is "noun is noun", where two things are being equated, then you want to use "is..." If you're describing, like "noun is adjective" or if the Irish would say "to have" (well, "to be at one") instead of "to be" in English, or if it can be rendered in English as "there is/are..." then you want to use "ta..."
Still not fixed.... will be reporting this one. Should have been "Lachain"
Why is the correct answer not "Is lachain iad.", when the translation of 'We are turtles' is given in the lesson as "Is turtair muid"? Just wondering...
The syntax is correct. The way Duo spells lachain is flawed. See the comments at the top of this page.