The Chinese expression "smiling face tiger" (笑面虎) is defined in English at one website as "an outwardly kind but inwardly cruel person".
More broadly it doesn't need to refer to someone who is actually positively cruel. The phrase might be used to refer to a person who superficially appears nice/gracious/easygoing/etc., but actually is not at all nice/gracious/easygoing/etc.
Any similar expression/word/idiom in English?
Edit to elaborate: One suggested answer in "wolf in sheep's clothing". This is perhaps the closest English translation (especially since it involves animals) but I think this English/European idiom has an emphasis on the fact that the wolf is deliberately disguising himself in sheep's clothing, in order to deceive someone and achieve some malicious end.
What I am thinking of is someone less malicious than a "wolf in sheep's clothing" (but can nonetheless--though not usually--be just as harmful). Examples:
He is always smiling, greeting you, and saying "Good morning, how do you do?" and generally being absolutely courteous.
When he spills some coffee on your shirt he might say, "Oh my goodness! I am SO terribly sorry! I am such an absolute and horrible klutz! Please let me wash your shirt for you!" (but with no intent of actually doing so, knowing that you will probably reject the offer).
He often offers to help you out or do you favors (again with no intent of doing so).
When you inadvertently cause him some inconvenience, he might say "Oh that is no problem at all! Absolutely no problem at all!" and keep smiling and seem unperturbed the whole time (but in fact he is extremely peeved and if you provoke him just a little bit more, he is liable to abruptly erupt in fury).
These are just some examples. You can probably think of more.
Such a person does this sort of thing on an everyday basis. Unlike a "wolf in sheep's clothing", he has no malicious intent to disguise himself or to seriously harm anyone else. Nonetheless, he has somehow acquired the art of delivering a very favorable first impression and being very nice with his words and superficial acts. But when a problem actually surfaces, it is quickly revealed that he is not very nice after all.