Answering my own question as I think this may be useful for someone else too.
CALC :
In LibreOffice Calc go to Menu "Tools" > "Options", then "Libre Office Calc" > "Formula" and check on "Use English function names".
Then also set "Formula syntax" to "Excel A1", this will make formula syntax more compliant to Excel "default form", for example, for the needing of changing . to ! in formulas for worksheets reference separator (that means Sheet1.A1 will become Sheet1!A1).
Lastly, if your language/locale uses , as decimal separator (like for Italian) then the above will not suffice, because the argument separator in formulas may yet be ; instead of , (for instance SUM(A1;A2) instead of SUM(A1,A2)) so, as final suggestion to solve even this, I highly recommend to go to Menu "Tools" > "Options", then "Language Settings" > "Languages" and change "Locale setting" to American/English as well.
(American/English locale should always be available with a standard installation of LibreOffice, otherwise uninstall+reinstall it if necessary.)
P.S. : If you want things restored back, simply do the inverse process and you're done.
EXCEL :
While it is free feature in Calc, switching language/locale may cost you around 20€/25$ with MS Excel since all MS Office major languages packs are not for free (for example: English Language Pack for Office 2010, English Language Pack for Office 2013).
It might be, some of the MS Office free language pack uses english-named-functions and you may try installing one to switch just for copy/paste necessities and then switch back to your language.
In the end, otherwise, you could try this Excel Multilingual Formula Translator Addin but it's quite dated and thus it is very likely it may not work on earlier versions of MS Excel.