Today someone used the word 'adieu' IG, which is a French word (the person actually typed 'adu' and came with some story about it being used all throughout the Baldur's Gate game, confirming several times that the spelling is 'adu', which I have never heard and is not an actual word in the English language - at any rate, if this turns out to be true, I am wrong and this topic can be disregarded).
Basically, I find this to be immersion-breaking. The French language does not appear in FR, and on top of that 'adieu' literally means 'see you at god' which is a figure of speech that is entirely inappropriate in regards to the FR religion system.
I'd like to know what the official DM stance is on using words from other languages than English in your speech when the character is speaking Common, this of course only in reference to words that are -actually- foreign and are not in common use (like, for instance, madame or its abbreviated form ma'am, which is also originally French).