@seadowg put in a pull request at https://github.com/opendatakit/collect/pull/3602 and I wanted to report back on some discussion that we had there. There were a few things I had not considered when reviewing this thread that came to me in code review.
The current proposal is for the dialog text to read simply Add "<repeat-name>"?. The "new" or "another" has caused confusion among enumerators and @seadowg has realized it doesn't add anything. In the case of repeats named with singular nouns, this will make very comfortable queries such as Add "observation"? or Add "Patient"?. If a form design uses a plural noun, it's not great (e.g. Add "Observations"? but it's no worse than it was before. This is in line with what @mathieubossaert had specifically requested here.
I can confirm that this will work well in Romance languages (e.g. Ajouter « mammifère »?, Agregar "fruta"?). @seadowg is adding a note to translators to make it clear that the goal is for a short sentence that works with any kind of singular noun (feminine/masculine or whatever other forms a language may have).
In the mockups above, @seadowg had used No and Yes for the buttons. We went back to general UX guidelines and found that verbs are generally recommended. Using verbs means that someone doesn't need to fully read the dialog to take action. Understanding what No and Yes refer to requires more thought. So the current proposal is to use Do not add and Add for the button text.
We hope to alert translators of the change in the next couple of days and to have a beta available soon.