I agree with you. I've never heard the expression "so fun". "Such fun" or "so much fun", yes. While I am really enjoying the Duolingo Gaelic course some of the English translations are really strange and dare I say a bit confusing at times. Languages are usually not translated literally but rather into the equivalent structures in the alternative languages.
I think it’s worth noting that Gaelic and English structure and tenses don’t always line up and there are various structures that we need to account for in future that do not yet appear. For example we have to keep “I am seeing / I see” separate because there is a difference in Gaelic. I have no problem with “so fun” being used. OED lists it as an adjective. It’s easily understood and reduces variations in Gaelic translations at this stage. We do accept “such fun” as an answer.
My comment is not about this it is about my inability to access the learning modules on my computer although i have been learning for over a year. Suddenly access stopped and I can't update my browser further. When i went back to my phone app it took me right back to the beginning of the course! Duo is very happy, i am not!