Internationalize all UI elements

This patch internationalizes the remaining (not yet internationalized) UI elements. Care was taken to properly escape all strings.

The only thing missing for a perfectly localized environment: 'N' and 'P' are hardcoded for next and previous image (German for example would naturally have 'N' and 'V'). But this is a minor issue.

BTW: I find your theme pretty impressive! Thanks for sharing it!