Hi @rorycb ,
having dual line titles with the original text could help in case the translation is not correct. And believe me it often happens. Anyway, I agree with you, there may have some users who prefer having only one title line. The second line could be an option to be activated in the user preference page.
The actual translator-plugin already use caching, so that you pay once for the same translation. Translation services are not totally free. After a certain amount of translated words you have to pay.
Data storage could be an issue depending on the forum size. In my case is not relevant.