In Agatha Christie’s Hercule Poirot detective stories, Poirot uses the phrase “if you please” a lot. Does this come from the French phrase “s’il vous plaît”?
Hercule Poirot is a Belgian and speaks Belgian French. In the television series, David Suchet uses and switches between if you please and s'il vous plaît on a regular basis, and as such wears his civilised foreignness openly. I expect that the Poirot in the books shares the same affectation.
So, yes, I do believe that the use of if you please is intentional and is the English counterpart of the formal s'il vous plaît in French.
Wiktionary's entry for s'il vous plaît reads:
s’il vous plaît ("if it pleases you")
(formal) please, if you please
Pourriez-vous me donner le pain, s'il vous plaît?
The Wikipedia entry for If You Please refers to a French play:
If You Please (S'il Vous Plaît) is a Dada–Surrealist play co-written by the French surrealist writer and theorist André Breton and poet and novelist Philippe Soupault.
S’il vous plaît means if it please you, not if you please.
The formulation if it please occurs often enough in the KJV.
If it please the king, let there go a royal commandment from him, and let it be written among the laws of the Persians and the Medes, that it be not altered, That Vashti come no more before king Ahasuerus; and let the king give her royal estate to another that is better than she.
However, if you please does not. On the other hand, if you please does occur 18 times in Shakespeare.
It doesn't seem that these phrases are related, because literally "s'il vous plait" means "if it pleases you":
si means "if"
il means "he" or "it" (as the subject of the sentence)
The word vous can function as the nominative "you" (as in "you stand here"), but in this case it's the objective case: "it pleases you"
plait is in the 3rd person: "pleases". For example, vous me plaisez means "I like you" (literally: "you please me").