On macOS it seems usual to ask the user a (bold written) question in a confirmation dialog and add a more detailed text (non-bold and slightly smaller font). Regarding the bold question - should it be a complete sentence, e.g. "Do you want to delete file 'foo.txt'?", or would it be fine to write an incomplete sentence, e.g. "Delete file 'foo.txt'?"?
Since you're asking specifically about macOS, I'd recommend you use Apple's macOS Human Interface Guidelines as your primary reference. See these relevant excerpts (emphasis Apple's):
Provide a message that describes the situation clearly and succinctly. A message like “An error occurred” is mystifying and likely to annoy people. Be complete and specific, without being verbose. When possible, identify the error that occurred, the document or file it occurred in, and why it occurred.
Consider phrasing a message as a question when the default action has negative consequences. For example, a question such as “Are you sure you want to clear the history?” pinpoints the action that produced the alert and encourages the user to consider the results. Don’t overuse this type of alert, however. Users tire quickly of being asked if they’re sure they want to do something.
Supplement your alert message with informative text. Use informative text to expand on the message text by elaborating on consequences and suggesting a solution or alternative. Give as much information as necessary to explain why the user should care about the situation. When appropriate, remind users when an action can't be undone. Whenever possible, suggest how to fix a problem. For example, when the Finder can’t use the user’s input to rename a file, it tells them to try using fewer characters or avoid including punctuation marks.
Consistency is important, though these are only guidelines—it's okay to not follow them entirely, but make sure you have a good reason to break from the expectation should you choose to do so. From the examples they provide, it seems that Apple favors complete sentences, so I'd probably try to stick to that.