The subject and message of the email that is sent when the Dropbox backup (or another service) is done are hard coded and untranslatable.
I think it can be changed to use the translation tag or add to Dropbox Settings DocType a subject and message variable (both success and error) so the user can customize the email.
Any suggestions o possible solutions? the most important is the user receives the email in his own language.
I can contribute with a PR of course.
Yes, the email should be translated. Feel free to send a PR!
There is one more issue/suggestion
Email message should contain default company or site name, so if anyone have more than one backup configured, one can easily understand which site backup are taken.