For my fantasy novel, I created the skeletons of three conlangs which, in the text itself, I use only as naming languages. That is, they appear only in names of cities, rivers, people, animals, whatever.
I'm doing something similar, but I'm using Dutch-derived words in a particular region of my world. For example, I have a remote village called "Verlaten," which means lonely or desolate. Etc. (Why Dutch? Because...if ya ain't Dutch, ya ain't much. Actually, I come from a heavily Dutch-settled area of the US, and I thought it would be a nice homage. Maybe I'll be really cute and have an inn called "The Wooden Shoe," or something.)
I also have a society of mages who are the literati in my world, and I'm using Greek-derived words when I need to make up a word associated with them. I have
amphorsophs and
amphitope. Because Greek-derived words sound scientific and/or philosophical.
But I'm certainly no linguist, and I don't speak Dutch or Greek. I just have an internet connection, so I hope I'm not messing anything up too much, and that all of this doesn't end up sounding silly. My theory behind all this is that, if you're building a completely fictional fantasy world, the characters aren't speaking English, or German, or any other earthly language. They're speaking whatever language they speak, and all of the dialogue is "translated." So, I think using real-life foreign languages is as acceptable as writing the book in English in the first place.
Some of my characters have accents, brittish, southern, french etc. I thought foreign languages just bring out the character.
I'm curious how you're going about that. Are you spelling the dialogue out phonetically? Or are you saying something like, "Pepe spoke with a French accent"? I think it could work in some situations, but not others. I think if you're writing a traditional high fantasy novel, having characters speak in German or with French accents would seem odd. But it would probably work in something less traditional.