"ʻO wai ka mea e uku i ka uku lawelawe?"
Translation:Who will pay the tip?
In this construction, mea can be dropped but ka or ke is needed. You are saying - Who is the one to pay the tip?
There is another way to phrase this sentence that is less verbose - Na wai e uku i ka uku lawelawe? In this case, ka mea is completely unnecessary.
(Literally, this would be "Who is the person to pay the tip? or Who is the one to pay the tip?" ) = Who will pay the tip?
Translations can never be an exact match from one language to another. but "ka mea" is necessary.
EA = [PE] 1 n Thing, person, matter, stuff, object. Cf. mea ʻole, what 3. Ka mea e loaʻa ana, whatever is gotten, found; anything. ʻO wai ka mea e ʻaʻa e haʻi ʻōlelo? Who will volunteer to make a speech? Hāʻawi mai i ka mea keʻokeʻo, give me the white one. Nā mea āu i noi mai, whatever you asked. He aha ia mea? What difference does it make? Ka mea hea? Which one? Which? Which person? Nā mea ʻelua, two things, both. Nā mea like ʻole, varied things, miscellaneous. [PPN meʻa] (link) https://manomano.io/definition/24976