I'm one of those people, I confess, who breaks up flow with long sentences of foreign dialogue. I try not to do it unless the dialogue itself is important to the story, and I provide footnote translations of the spoken words as needed. I find that sometimes, the fact that a foreign language is being spoken and not always understood by other characters present can be a very important part of a scene.

When I use foreign dialogue, it is either from a language I speak myself or have consulted with native speakers. You're right that it's critical to understand what you're referencing and use the language and culture both correctly and respectfully.

My problem with the occasional dropping of a "Merde!" is that it doesn't say that much about a character... other than that they know how to exclaim "shit" in French. If my characters are speaking a foreign tongue, it has significance--enough that I want my readers to remember that moment or content at a later date. I use foreign language as a punctuation mark, as a way to draw a reader's attention. A contributing factor is that I write a number of Viktor Krum-style characters, folks with a limited English vocabulary, potentially biased view-point and strong cultural background. When my Krum speaks Bulgarian, it's because he's muttering something highly inappropriate to a teammate and doesn't want anyone to know what he's talking about because of the potential media scandal. He's not being cute--he's being devious and secretive while covering his own ass. That says something about the inner workings of my Krum character.

So foreign language can be used to add color to characters, to fill in characterization gaps, or to advance plot. I think the way we present a foreign language in the text says a lot about what the ultimate aim is.