Butter is not quite countable in Romanian either.
But we don't normally say "a stick of butter". At most we refer to it as "un pachet de unt" ("a packet of butter"). However, we most often drop "pachet de" and therefore get "un unt". If there's more than one, we count "pachet" (neutral noun), not the butter itself.
It's the same with coffee and tea: "o cafea" means "o cană de cafea" ("a cup of coffee").
sure i guess it's possible you could say "i have an old butter" but it sounds really weird to my ear. i would always say "an old stick of butter" or (like you) "an old tub of butter" or (if it's a factory) "an old 1000 gallon vat of butter." but it's not easy to think of a reason to say "an old butter" without its coming across as contrived.