For example, I'm editing a book that uses French terms for family members. If they are using Grand-mère, Grand-père, Maman, and Papa to refer to their relatives, when should they be capitalized and when should they be lowercase? Does it follow the same rules as English where you use lowercase when saying something like, "My mom says..." and uppercase when saying, "Mom says..."? Or is it different? And when capitalizing Grand-mère and Grand-père, do you capitalize it like I just did, or do you do it like Grand-Mère and Grand-Père?
In French you will capitalize those only if used as proper nouns, but that is not a very big mistake. What you must do though, is to accentuate all the letters that need to be, like, grand-mère (or Grand-mère. Note that kids would rather call their grandma Mamie or Mémé than Grand-mère when talking to them or referring to them.– NoneFeb 18, 2020 at 8:41
@None it pretty much differs from one family to another, and maybe even from a region to another. My daughter calls my father "papy" and the father of my wife "grand-père". I myself used to call my grand-father "papy" while referring to him as "Mon grand-père".– Laurent S.Feb 18, 2020 at 16:29
A leading uppercase is always mandatory when a word starts a sentence.
Papa, tu es là ?
Otherwise, papa, maman and the likes are noms communs so normally take no leading uppercase.
Quoi, maman, vous n'étiez pas sage ?
Charles Beaudelaire, Les Épaves, 1866.
However, you are also free to use what is called the majuscule de déférence.
Vous êtes là, Papa ?
Finally, the first letter is generally the only one in uppercase:
Grand-mère, tu es là ?
although there are a few cases of Grand-Mère and Grand-Père, either because it is a proper noun like a brand name, or because of the deference case.
You are right and should capitalize only words used as proper nouns, and in the given examples, only the first letter is capitalized.
Also don’t forget the accents, including on capitalized letters.