I was browsing the internet, and found that "Please do the needful" is not an appropriate sentence to use or write.
According to this link, this sentence used to get used in South Asia. What would be an alternative phrasing to use?
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From my experience of receiving such requests from colleagues in India, I would say that the idiomatic equivalent is either Please help, or Thank you. There really isn't a literal equivalent that anybody would write in the UK, in the same way that there isn't a British English equivalent of bon appétit.
Close equivalents might be "please do the required" or "please do what is needed" or "please do the necessary"; all imply a certain background knowledge of the situation, i.e. that you know what do to and don't need it spelled out.
I'm not sure that "do the needful" isn't entirely inappropriate, though (I'm in the UK). In that way that certain archaic phrases are used in a slightly jocular ways, e.g. using beverage instead of drink, it does sound lighter and less formal and probably something to be spoken, not written.
As @Mitch has pointed out in his comment to the question, there is considerable discussion about the expression Please do the needful here.
The answers there, however, discuss two - what I consider to be different - meanings for that expression:
Do the necessary or Do what is needed.
I consider this to mean Take the appropriate action to achieve the required result.
For example (quoting @ManishSinha, the author of the linked question):
"Please do what we expect you to do in this situation without being provided a detailed explanation"
"For example, a project manager gets a mail from marketing or quality assurance about something missing or incomplete, then he/she sends a mail to the subordinate with the body "Please do the needful". The subordinate is usually more well-versed with the work which has to be done. It might be his/her expertise so the manager might not tell what exactly needs to be done as is left upon him/her to figure out."
Do what's right or Do the right thing.
The expressions in the line above are also quoted from the author of the linked question, but (to me) those expressions mean something different from my first meaning.
I always considered these expressions to refer more to 'etiquette' or 'politeness'. This is not a case of 'taking the appropriate action to achieve the desired end', but a case of responding in the socially accepted 'polite' manner.
So, in my view, the answer to your question actually depends on its usage is India. Can it mean both of the above, or only one of the above?