Do you have to explain why you're using German and Latin names in a novel? Let's say it's a fantasy setting and you use German and Latin names for your cities. Do you have to explain any of it, or you can do whatever you want. What's the best way to go about this, because it seems some novels don't comment on the origin of the words and use German and Latin name even if it doesn't really make sense in their fantasy world.
No, But Be Careful When Using Invocation:
I can't really dispute Sciborg's answer, but I do think it deserves an addendum. The use of German or Latin place names, characters, etc. can be completely coincidental, OR it can be used intentionally to create color for your setting. Using names of a places and people from a certain culture tends to evoke thoughts and feelings about that culture in the minds of your readers.
Like it or not, a bunch of Latin names will evoke the Roman empire, or possibly the old Catholic church. French will evoke bards and minstrels, Japanese will evoke shoguns and samurai, and German has a complex history I'm personally well aware of and evoke as needed to create a setting. So if you evoke a certain language, you create an expectation in your readers that the society those words are used in will in some ways match either the true culture OR a stereotype of that culture that those words represent.
If you are unaware of this effect, you will offend people who may identify with that culture, or be dismissed by people who feel you are ignorant of the culture you are evoking. You are also missing a golden opportunity to add a rich layer of color to your story if you fail to exploit this as you write. Be aware of the flavor of the setting you are creating, and add these words and names to create a world where people already anticipate the setting. But if you do this, be well schooled in the deeper meanings of the invocation you are casting. Nothing is worse for a word wizard than to cast a spell with words and have it go awry like the Sorcerer's Apprentice.
As with most things when it comes to writing, you are the author, and you can make whatever worldbuilding decisions you want without having to justify all of them. If your characters all have traditionally German and Latin names, then that's what they have. If they have Asian, African or French names, that's what they have. That's all there is to it.
This is somewhat related to the "all of the characters speak English" problem that comes up in fantasy writing. In a lot of fantasy novels, there is no equivalent real-world methodology by which the English language would develop in the world, and yet all the characters speak English and name things with English words ("the City of Tears", "the Last Dragon", etc.). But the readers don't really think about that or care, because having every single character speak a deliberately confusing, non-English language that fits with the setting and that the reader has to learn to understand the novel would be overly demanding of the reader. So we have a certain suspension of disbelief that comes into play that allows us to say, "well, there's no way the development of the English language could really make sense in this world, but the novel is written in English for my enjoyment, so I can overlook it." (Some authors will also imply that the characters are speaking a non-English language, but it's just being translated into English for the sake of the reader, which is an equally valid approach.)
All of this is to say that you don't need to explain every minute naming choice you make, and most authors don't unless it is directly relevant to the plot at hand, because it would just bog the novel down with unnecessary details.