I think the fact that Antonia speaks her native language simply supports the theme that through all of her experiences, good and bad, joyful and devastating, that she has remained the person she always was beneath the skin. This young woman never lost sight of who she was, and Jim, the narrator, never quite seems to find or understand who he is.
At the end, though she is now a woman, worn by the sun, hard work, and her experiences, she is still a fountain from which springs energy, passion for the land, and love for her own. She has not forgotten her second language, but more so, she remembers where she comes from, and part of that is her father, who she admired and adored.
By the novel's end, Antonia has come full circle. She is comfortable and fulfilled, with a complete sense of herself, of which her Bohemian heritage is a large part. By speaking with her husband and children in the Bohemian language, Antonia is embracing her past, affirming the influence of the land of her birth.
The theme of the connectedness of all elements of life and the circular movement of the seasons and life and death is central to the novel. Antonia's return to the language of her birth emphasizes her own harmony with the natural cycles of living. Her roots, past experiences, and present situation are integrated into her selfhood; she is a whole person, and wonderfully complete. Seeing how happy she is raising her family in the country, Jim comments that she "ought never to have gone to town", but Antonia treasures the hard lessons she learned from her time away from the land as well. She says, "Oh, I'm glad I went...I've been able to bring my children up so much better...if it hadn't been for what Mrs. Harling taught me, I expect I'd have brought them up like wild rabbits...I'm glad I had a chance to learn" (Bk.V, Ch.II).
Antonia is the sum total of all the parts of her life and experience. Her Bohemian heritage, as exemplified by her retention and use of the language, is a huge part of who she is, and will be passed down to future generations through her children.