What is the equivalent proverb/idiom for Hindi saying in English?

Solution 1:

Two-faced

Someone who is two-faced is not sincere, saying unpleasant things about you to other people while seeming to be pleasant when they are with you.

(From the Cambridge Dictionary)

which I think is the most suitable in OP's context.

Solution 2:

The closest I can think of is a proverb for a subset of such behaviour where the smiling and stabbing actually take place at the same time, but the hypocracy at least is intact:

damning with faint praise. From Collins Dictionary:

If someone damns something with faint praise, they say something about it which sounds quite nice but is not enthusiastic, and shows that they do not have a high opinion of it.
Dominique damned it with faint praise :'It tastes quite good when you've lived in the UK for 22 years'.

Wikipedia gives the following origin for the phrase:

The explicit phrasing of the modern English idiomic expression was first published by Alexander Pope in his 1734 poem, "Epistle to Dr Arbuthnot" in Prologue to the Satires.[4]

    Damn with faint praise, assent with civil leer,
    And without sneering, teach the rest to sneer;
    Willing to wound, and yet afraid to strike,
    Just hint a fault, and hesitate dislike.

            — "Epistle to Dr Arbuthnot" by Alexander Pope (1688–1744)[5]