Learn English – drop the /r/ sound if it’s in the middle of a word

If you mean the difference between the two L’s in the word little, the first one [l] is the same as the Italian L of your acquaintance and is sometimes called a “bright L” or a “clear L”, while the second is an allophone of English sometimes referred to as a “dark L”, written [lˠ] or [ɫ].

The key difference is that the dark version is velarized, which here means that the back of your tongue flattens out and moves up a bit. The normal demo-word for this sound is FULL. If you have ever heard Catalan spoken, you will find that the Catalans often have dark L’s where a Castilian speaker would have a bright one like you do. It probably sounds a bit “swallowed” to your ear.

The exact distribution of these two allophones for phonemic /l/ varies by region, speaker, and syllable position. Some speakers have only one or the other of the two, while others have both. ^[reference]

Technically speaking, the dark L is:

The velarized alveolar lateral approximant, also known as dark l, is a type of consonantal sound used in some spoken languages. The regular symbol in the International Phonetic Alphabet that represents this sound is ⟨lˠ⟩, though the dedicated letter ⟨ɫ⟩ is perhaps more common.

In contrast, the other L is:

The alveolar lateral approximant, also known as clear l, is a type of consonantal sound used in some spoken languages. The symbol in the International Phonetic Alphabet that represents dental, alveolar, and postalveolar lateral approximants is ⟨l⟩, and the equivalent X-SAMPA symbol is l.

I'm... glad you asked this question. English has a strange way of taking words from other languages and punching them into shape. As with every last rule in English, there are exceptions and special cases to pronouncing French words in English.

Most of it (or so I believe) boils down to convenience. People prefer 'Parriss' instead of 'Parree' simply because the spelling has an 's' on the end, while they will happily not pronounce the 't' in 'ballet'. The only way I can attempt to shed some light on this is to go through every example you've set.

'Renaissance' is its own word in English with its own definition. In this case, you would pronounce it in an English way. Or, for someone who enjoys French, no one would mind a French pronunciation as long as it was accompanied by a heavy English accent.

'Deja vu' is a phrase we stole. 'Deja' isn't in a standard English dictionary, and neither is 'vu'. Thus, you would pronounce it as a Frenchman. 'Par excellence' is also a phrase, so a French feel is the best way to approach it.

History is slightly more difficult. I am no expert, so I'd recommend asking another teacher / historian etc for specific words. I can say with confidence though that 'Versailles' is pronounced in a very French way, as it would sound nasty in an English accent.

'Les Miserables' is part of the culture of Britannia. Everyone is aware of it. And, in English pronunciation, 'miserables' is hardly a fitting name for a show of such majesty (or so its media frenzy would suggest). Whenever culture (film, plays, books, people) is involved, chances are the pronunciation is French.

So... how to make this general. A big challenge for me. I would say: names, cultural references, words that would sound horrible in English, and phrases keep their native pronunciation; places, words we have adopted, and words that would be difficult to pronounce use English pronunciation.