I'd say it's more "authentic" to use the language at that would be used in the situation, if you are writing a realist piece, if you actually know those languages that is. Having somebody else translate something in from English into another language to just drop into a text is just obnoxious and serves no real purpose.
Another reason for having multi language writing is that different languages have different features, either inherently grammatical or in relation to the culture in which they are used, which is lost in translation, and can bring a subtleness which can't be conveyed in a different language. After all that's why doing good translations is difficult and people endlessly argue over translation.
Here's a small, simple example with English and French; in French there is two different words that are used for the second person pronoun, were we only have one, 'you'. No matter who you talk to, you always say you if you are talking to them; 'do you want something to eat?', 'would you like to leave?' Etc. But in French they have separate registers to indicate different levels of formality. When you talk to family members, close friends, children, people that are under you in your company or military you use 'tu' as the second person pronoun, but if you are talking to a stranger or causal associate, an elderly person, your boss etc, you use 'vous'. Vous also gets used as a plural second person pronoun, which English doesn't really have, we just awkwardly say 'you guys', 'y'all', or 'yous'. To some degree French speakers sometimes silently fret over whether to use vous or tu when talking to somebody, as to not come off either overly familiar or too distant. A lady might be put-off if you tu her too quickly, it's not particularly gentlemanly. The switch from one to the other could be seen as an important marker of growing familiarity or a change in a power dynamic between two people. English used to have this distinction, we had both 'ye, you, your', which was our formal register like French's 'vous', and we had the causal 'thou, thee, thy', like French's 'tu, toi, te'. So hopefully this shows a linguistic feature which, since it had no equivalent in English would just be totally lost in translation. This is a pretty small example and really is just the tip of the iceberg so if an author knew a couple languages and really wanted to convey a specific scene in a way that was authentic to the culture it is depicting, using the language of the culture is absolutely necessary for portraying the subtleties that make the literature real.
That's a part of the reason why people advocate reading works in their original language, especially fine literature, language is integral to a cultural and a work of literature need to be understood to be in the context of the culture it was written to fully embrace the work and all that it is.