Hi Janet, it sounds great. I agree the 'second language' bit is a little clunky. How about '...raised on, Katherine immerses herself in her cultural heritage - the language, the beauty of Prague...' I'm not sure that you need to make it explicit that she is bilingual. You could try 'the language of her childhood', but that might be misleading if she still speaks it at home with her family. I think the general idea that she enjoys discovering more about her own roots stands, whether the reader knows she is bilingual or not.
I think 'But she also finds herself involved in a love triangle' could be stronger. Maybe something about how the love triangle disrupts the happiness that she is finding in exploring her heritage. It would be nice to get across the sense of what is threatened by the love triangle and the involvement in the resistance. Something like 'But the precarious peace she is building for herself is threatened when she becomes involved in a love triangle and dangerous covert resistance...'
Overall it sounds like a really compelling book. Can't wait to see when it's published.