This looks like an oversight, or possibly an unreachable localization (since it appears to be hardcoded within text-formatter). Nonetheless, I recommend you submit an issue on the s9e/text-formatter github so JoshyPHP is aware of it.
As a workaround, you can enable the Markdown extension and use > @username#postnumber quoted text for a different style quote without the unlocalized text. It appears as such:
