The original text reads:
: виростем! — сказали тополі.
: бризнем піснями — сказали квіти.
розіллємось! — сказав Дніпро.
Тополі, квіти, і Дніпро.
On the first two lines, the verbs (виростем, бризнем) are future imperfective forms. The third line though, розіллємось is not a form I can find in any conjugation charts¹. I suspect that it’s a coinage of Tychyna’s, although maybe someone who is a Ukrainian speaker can elucidate. Google translate offers a translation of “let’s spill”, treating the form as a first person plural imperative and that makes sense in the context of the whole stanza, and it seems reasonable to interpret the stanza as Dnipro² and the others not only referring to themselves, but to those surrounding them, so it’s not that the Dnipro is referring to itself in the plural, but rather that he’s inviting the poplars and flowers to overflow (metaphorically) along with him, much as the flowers are inviting the poplars and Dnipro to burst into song and the poplars are inviting the flowers and Dnipro to grow tall, each expressing their enthusiasm in their own way.
And a Google search for
розіллємось mostly turns up Tychyna’s poem!
The “old man” before Dnipro is an invention of the translator, by the way.