I submitted this as an official support question to Atlassian, and the answer was basically, no, there's no way to do that. Here's the text of the answer:
There is no way to change this label through the interface. I also tried the InProduct Translation but with no luck.
Maybe this can be achieved by some changes in the code, but since customisations are out of scope of this support channel, I'd advise you to ask it in a more specific way at answers.atlassian.com which is our community network where anyone can help you, and even our developers participate there.
Or, if this is an urgent matter, I can refer you to one of our Experts, that can make almost any customisations to JIRA.
Thanks for your understanding. Please let us know if is there anything else we can do for you.
You need to change all mentions of sprint into iteration in the BoardAction*.properties files.
Be sure to only change the values and not the keys.
For example :
gh.board.dispatch.error.no.boards.for.sprint=There are no boards which contain this sprint.
is changed to
gh.board.dispatch.error.no.boards.for.sprint=There are no boards which contain this iteration.
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