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?
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.