For starters, I would say, with narrow exceptions, write your dialog in one language. If you write some dialog in English and some in French, your book will only be comprehensible by people who know both languages.
You could give translations for everything said in the "other" language. But then why bother with the original language? Just say, "Bob asked, 'Where is Mary now?' in French." I've read many books and seen many movies set in foreign countries or even on other planets, where presumably the people do not speak English, but where all the characters in the story speak in English and it's just understood that this is a translation.
(I saw a movie made in Mexico once that included a scene where several characters travel to the United States and get in some trouble with the police. And in the movie, when the two American policemen are discussing the case, with no Mexicans present, they spoke in Spanish. I found this quite amusing. But I'm sure foreigners are disconcerted in the same way when they see an American movie where, say, two French people, in France, speak to each other in English.)
If there is some point in your story where some characteristic of the "true" language is important -- where a character uses a word that is difficult to translate, or quotes a piece of poetry or song lyrics or a well-known proverb -- then give THE MINIMUM AMOUNT of text in the original language, and provide a translation. If at all possible, give the translation in the body of the text as part of the story, not as a footnote.
I mean, absolutely do not write, "John said, 'Makakaon ako og napulo ka saging'", and expect an English-speaking reader to understand what is meant.
Second worst would be to have a footnote giving the translation. For one sentence, this might be tolerable. But if you have whole conversations with translations in the footnotes. this will be tedious and annoying.
If it really helps the story to talk about differences between languages, you could write, "John said, 'I can eat 10 bananas'. He used the conditional tense to indicate that he was capable of eating so many bananas, not that he actually had eaten that many or was about to eat that many."
But for the most part, just give all dialog in English, or whatever the primary language of your book is. Most readers who do not know French have no desire to hack through text in French and then have to read a translation. I'd only lapse into another language if it truly adds to the story in some way. 99.9% of the time, it won't. Just write in English.
Off the top of my head I can't think of any fiction stories I've read where some peculiarity of a foreign language helped the story.
I can think of one sort-of example from non-fiction: Many students of the Bible talk about how there are three different Greek words that are all translated "love" in English Bibles: agape, philos, and eros. Discussing which was used in the original in a given sentence and why can be instructive. And can help clear up puzzling translations, now and then. For example there's a well-known passage where Jesus says to Peter, "Peter, do you love me?", and Peter replies, "You know I love you", and Jesus is clearly not satisfied with this reply. This is puzzling until you read the original Greek and see that Jesus said, "Peter, do you agape me?" and Peter replied, "You know I philos you." "Philos" is a lesser form of love than "agape", so it would be like a woman asking her husband, "Do you love me?", and he replies, "Hey babe, you know I like you."
But frankly, if I was writing a novel in English, even if the characters would be understood to be speaking French (or Alderaanese or whatever), I'd make any subtleties of language be English subtleties. Because otherwise you have to explain for the non-French speakers, which is unnecessary surplussage cluttering up your story for those who know French, and probably tedious pedantry for those who don't.