For the image labels, you could publish a brief spec/howto of how you create the default labels, crowdsource translated versions and then pick the right one in the code based on current locale.

But then again I think simple text instead of images would work even better. I’m not sure what you mean when you list ”Style (e.g. prev/next buttons) cannot be customized”: surely that applies to images more than text, which can be bent any which way with CSS?