Removed 6 back-references from language files
Fix sprintf/printf confusion. Thanks to @Discordier
* removed more backreferences in language files (refs #1171)
* Removed last real backreference ($lng['dkim']['dkim_keylength']) from language files (refs #1171)
Removed error references strings from all language_files, these language files should now be suitable for translation using transifex (refs #1171)
Merge pull request #87 from madmuffin1/language_cleanup

Language cleanup, fixes #1171