"The majority of people eat meat."
Translation:Większość ludzi je mięso.
The verb here agrees with "majority", which is grammatically singular in Polish. So of course "Ludzie jedzą mięso", but "Większość ludzi je mięso".
...Ale gdyby nie jedliśmy mięsa, Ziemia dziękowałby nam. =) please correct me if there is mistakes in this sentence :)
ale gdybyśmy nie jedli mięsa/ ale jeśli nie jedlibyśmy .
also nam at the end- technically correct, but I would say Ziemia by nam dziękowała
also you must consider if you want Earth to keep thanking us, or just do it once. If one "thank you" is what you have in mind - podziękowała(by)
'osoba' is generally a bit difficult in usage. I'd say that it doesn't suit the sentence, because it has "the majority of people" (in the world? something very general) and not "the majority of people in this room". 'osób' would suggest some more specific persons.