An heuristics can add a proper markup in most cases (lists, headings…). I see two exceptions:

• Images: some textual image may want to be fully replaced, whereas we only want to translate the captions for pictures.
• Some Template parameters are string constants (no localization expected), whereas other one are literal content which should be translated. We could use templateData (string vs content parameter types), but those are rarely well provided.

To handle a better UX, what about rewriting this tool as an interactive JavaScript-only feature?
Edit: actually, it is already a JS feature. My bad.