We used to do this, and I am sure they still do, at la Iglesia de Cristo de lengua espanola de Genebra, Suisa. We’d start a sentence in Spanish and then switch to French. By the end of the sentence, or the paragraph, we would have switched back. Spanish was, of course, the primary language. But everyone there lived in French speaking Switzerland, so they also spoke French. There are some words I never learned in French because I knew them in Spanish and didn’t have to learn them.
I did not, however, learn much of Catalunan. The only expression I know in that is the translation for “dunce.” (I got called it way more often than needful.)