English has borrowed (or stolen?) a lot of vocabulary from other languages such as Latin, German, Italian, French and Spanish etc. Most words that are borrowed are anglicized and are pronounce the way English prefers. There are many examples out there.
But nowadays when English borrows words from other languages, it does not always anglicize those new words and pronounce them the way they are pronounced in the language they are borrowed from.
Examples: bon vivant: /ˌbɔ̃ːŋ viˈvɑ̃ːŋ/, mot juste: /ˌməʊ ˈʒuːst/, au fait: /ˌəʊ ˈfeɪ/, pas de deux: /ˌpɑː də ˈdɜː/, c'est la vie: /ˌseɪ lə ˈviː/
Most of them have some silent letters. There are many more.
Why doesn't English anglicize them and give them its own (anglicized) pronunciations? Is there any specific reason for that or it's just a respect for the language English borrow words from? Also, most of them are marked as "formal", why is that so?