Thanks for the reminder. The in-context translation is an issue raised often in moots and other places and we started to talk about it again in the HQ. Actual work on it may be a bit challenging without wider knowledge of Moodle development, but also a good opportunity to dive into it
Yes, I think it is doable cleanly via a plugin (a block or admin tool should fit well). Let the plugin provide a custom string manager class to track the strings used at the page, and then the interface on the page (I would prefer JS overlay rather than the footer these days) to translate strings.