So... imagine you have a story in your native language written down, with protagonists that speak this language and no problem at all, except a bunch of scenes where your protagonist meets someone who speaks another language. And you write it down in that language. I don't think this is a huge problem at all, even if you assume that your protagonist does not speak this tongue (but the reader may understand it) and you are using this as some kind of a plot element. Everything is fine until now.
Now the unexpected happens: your story is kind of successful and gets published in your country. Now it even amasses more fame and suddenly someone gets the idea to publish it in other countries too... so what should be done with these dual language scenes, especially if it got published in even that language your protagonist has no clue of?
What starts off as
Poor Siegfried was standing around, listening to the foreigner speaking "Ha, der versteht uns doch sowieso nicht, der Depp"
will get to
Der arme Siegfried stand herum, hörte dem Gespräch der Fremden zu "Ha, der versteht uns doch eh nicht"
or (inverse it)
Poor Siegfried was standing around, listening to the foreigner speaking "Ha, he will not understand what we are saying, that moron"
I mean, you can't just flip the language, if you state that this did happen in country whatever before, because readers will get confused if the native language in Germany is suddenly declared to English. For example.
Is there any way to avoid this kind of pitfall, aside from writing
They said in their native German language "that moron will not understand what we are saying" and poor Siegfried indeed did not.