Atlassian used to have a document on "customising text", but they've deleted it and seem to have removed the content (not just absorbed it into another page), presumably to strongly discourage you from doing this sort of thing.
JIRA uses a language file to generate all the data you see on the front end, and you used to be able to hack that to change things like this. This approach seems to be changing, which is why I think they've killed off the original page. The other route that is still open is to use a "translation" and translate Jira's default English into an English suitable for your usage - see https://confluence.atlassian.com/display/JIRA/Translating+JIRA
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