This isn’t a Kleo specific question, so you’d be better off asking for assistance on the bbpress forum. With that said, the common way to do this is by translating it. Dont be confused thinking you need to translate to a different language. Translations can be done in English if you merely want to substitute word. You’ll have to translate bbpress and buddypress (and there may be a few in Kleo as well). Here’s some useful links
https://translate.wordpress.org/projects
https://codex.bbpress.org/getting-started/bbpress-in-your-language/
https://codex.buddypress.org/getting-started/customizing/customizing-labels-messages-and-urls/
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