There are thousands of languages in the world and thus I'm suspecting there may be millions of possible sound combinations that work for this, and just based on a quick jaunt through the foreign-language sections of Wiktionary (credit to all the editors who helped create it!), I think I may have uncovered one:
Explanations are below.
"In some cultures I am something that all people experience in their lifetimes":
moray is a homophone of the Welsh word more, which means "morning" -- virtually all of us will experience at least one morning
"In other cultures I am something most people have no personal familiarity with":
moray in English refers to an eel -- a creature that many of us will never meet.
"In one language I am only commonly used by people who study science":
Also in English (although this is a different spelling, so I think it counts), moray can be pronounced the same as morae (plural of mora), which is a term from linguistics that refers to syllables or metrical feet
"In another language I am something of importance to everyone":
moray is a homophone of more in Serbo-Croatian, Slovak, and a few other languages, which means "sea" -- obviously, the sea is important to all life on Earth.
As mentioned, this probably isn't the answer you were thinking of, and it's definitely not the only one. In other words, there are many, many...
EDIT: By the way, if all four homophones need to be in different languages, an alternative answer for the "science" clue is:
more in Dutch, which refers to "the unit of length (short or long) in poetic meter" -- again, most likely used only by researchers of poetry and linguists.