I'd pass on this if I were you. At $12 a month that's already more than half of the cost of a claude.ai subscription, and even after just a couple days working on your prompt engineering, I imagine you could get Sonnet to generate stuff just as good without the restrictions this app has.
That said, I don't know if any AI model is currently trained on a large enough body of text in Catalan to give you stuff that doesn't have a massive hallucination rate.
The funny thing is the ads I've been seeing lately have been saying Krashen's model is out of date and are encouraging people to work on their output instead with their chatbots.