Users with proper capabilities can click "turn editing on" to get a course page with extra items allowing them to edit the content. For language editing, there could be a parallel button "show language strings" ("hide language strings" in the other state) showing on every page, if admin activates it in the site settings and a given user has the required capability (cf MDL-10576).

When this button is clicked, Moodle would continue to work as before but string replacements would be suppressed, so all strings would show as their names in double brackets, that is [ [stringname] ] (which now happens only when a string is missing). This would let translators, those who work on language packs as well as people editing local language extensions or local language packs, to easily toggle between the two views and clearly see which string is used where.

I don't think this would break anything operationally and should be doable without major effort. I believe we would just need extra code in the function which is doing the string substitution and extra function in the header which calls a library function to insert the button if needed (and toggle its text).