I just wanted to point out that "no" in Swahili is hapana, not hakuna. Both are structurally identical, differing only in the class of the subject prefix. Class 16, with pa- generally refers to a precise location whereas ku-, class 17, refers to an imprecise location. Hakuna [ha-ku-na :: NEG-C17.SUBJ-COM] is used to mean "There isn't/aren't (a/any) ...". Hapana [ha-pa-na :: NEG-C16.SUBJ-COM] means "There isn't/aren't (a/any) ..." at a specific location.
Hapa hapana tembo. = There isn't an elephant (right) here.
Huku hakuna tembo. = There aren't any elephants around here.
Hakuna is likely to be used as "no" only after questions with kuna "is/are there"?
Je, kuna tembo huku? = Are there elephants around here?
Hakuna. = No, there aren't.
Hapana is possible with other sentence types.
Note, however, that in most discourse in English, people don't usually just answer with "yes" and "no". Of course it's possible, but it has a rather abrupt tone. We could regard English as having over 50 different ways to say "no", all of them around 3 or 4 syllables in length and agreeing with the subject, auxiliary verb and tense of the preceding question: No, I'm not. No it didn't. No, we can't. No, there wasn't. No, they won't. No, it hasn't. No, you shouldn't. No, she doesn't. ...
Another thing to note is that ndiyo is only two syllables. The nd is a single phoneme, a prenasalised voiced alveolar stop. The y also doesn't stand for a phoneme because /i.o/ and /i.jo/ are not distinguished in Swahili. For this reason, a lot of people misspell it as ndio (which is also a valid word). The difference between ndio and ndiyo is class agreement. Ndio uses a C2, C3, C11 or C14 morpheme -o, whereas ndiyo uses -yo, a C3, C6, or C9 morpheme. In environments after /a o u/, the difference between /jo/ and /o/ is clear (as in mti unaOkua "a tree that is growing" vs. miti inaYOkua), but not after /e/ or /i/. The spelling of ndiyo for "no" is based on the fact that it uses class 9 agreement and there is no rule for spelling change that makes this drop in environments where it cannot be distinguished. So, phonemically, it's /ˈⁿdi.o/ — two simple open syllables, only one consonant.
I had a look through the translations for "no" on Wikipedia and didn't see anything longer than hapana as the only translation. There are things like minimē in Latin, which is as long, or longer owing to the long vowel, but there are shorter versions. If you don't accept "affirmative" and "negative" then I suppose you wouldn't accept minimē. I find it unlikely to find a longer word or phrase that you would accept because there are always likely to be shorter versions.
Even in Swahili, hapana is not the shortest way to say "no". That would be la. There is also siyo (again, with /j/ only distinguished in orthography, resulting in many spelling it as "sio"), which is the negative equivalent of ndiyo. Hapana is just the one that's widely used and widely taught, given as the most basic form.
Another thing to consider is that languages with simpler syllable structures and fewer phonemes tend to have higher syllable counts, but those syllables are, on average, uttered at a faster rate.