Filed:
https://bugs.launchpad.net/bugs/1514468
Merged in source:
https://github.com/kovidgoyal/calibr...5013c8b05e0df6
https://github.com/kovidgoyal/calibr...b71951db1ad983
So now default prefixes depend on the type of file, which should help.
e.g. renaming a bunch of images will default to "Image-" instead of "Chapter-"
It still won't remember the last custom prefix you used though.
Hopefully this is enough to mitigate the problem.
And as I said, they are all translated -- actually, the new ones will need to have translations added on Transifex first -- so foreign-language users shouldn't feel left out.

(If you want to use the English GUI I guess it makes sense for calibre to offer the English prefixes...)