Resolution: Not a bug
When translating looking at a Moodle context, for example a block settings, some strings may appear in English language since they are not translated yet. Strings seen in the same context may come form several language component (think at the course settings page).
When a untranslated string appears in a Moodle context but is taken from a component different from the component currently in translation, would be very useful to have a tool allowing direct string search and related component in the English language pack.
Such a tool would be very good for in-context translation, which is much better than direct component translation. I think would suffice a flag close to "Substring" search field; when checked, Substring search could return component(s) where strings is contained as well as strings.