In canon, Louis doesn’t speak a lot in French (sometimes the narrative will indicate the characters have switched to another language but the writing will continue in English for the benefit of the reader). He does throw in a mon dieu! (my god!) here and there. He says it when Lestat first picks him up to show him he has the Cloud Gift in QOTD.

It’s fanon that Louis and Lestat still speak to each other in French, an older version of it that mortals usually aren’t all that familiar with. Lestat still remembers a few old French songs from his childhood.

Louis is actually French, his family moved to New Orleans from France before he took over the plantation from his father: “You see, we lived far better [in Louisiana] than we could have ever lived in France.” – IWTV. So French was most likely his native language.

He gives the interview in English because that’s the language Daniel speaks, and Louis must have learned it at some point, probably in his travels with Armand. But the languages he probably spoke in New Orleans during his ~70 years there were French and Creole, and then when he goes to Paris, he would have spoken the more classic French and he probably had a noticeably Americanized accent, yet another reason for them to dislike him bc TOURISTS.

Bonus:

I found this fun Creole Dictionary online, check it out!

Here’s a sample of Creole compared to plain French:

English: I will never abandon you.

Creole: Mo va jamé bandonné twa.

French: Je ne t’abandonnerai jamais.

[French speakers are welcome to correct me on this, but this is my translation of the above for the sake of comparison]

In the Creole, it seems more casual, but still conveys the feeling: “Me gonna neva’ ‘bandon ya.”

• “Mo” seems to be like “Moi” which is “Me” or “myself" in French.
• “Va” the verb “to go,” but it’s used before a lot of verbs, “bandonné” is “to abandon” so it’s “going to abandon” when you put them together.
•  "jamé” seems to be like the French “jamais” which means “never.”
• “twa” seems to be like “Toi” which is the informal “you” in French.

In the French, it’s more direct, and classier: "I would not abandon you ever" basically.

• “Je” is “I”
• “ne” is part of the “jamais,” meaning “never”
• “t’abandonnerai” this is the “would” form of “abandon” plus the “t’” for the informal “you.” So it’s “would abandon you” all put together.
• “jamais,” meaning “never”