How to translate the "Confirm Contribution" button on Contribution pages
Multilingual word replacement support is the topic of both CRM-14349 and CRM-19683.  CRM-19683 just had a patch merged, which should be available in the January 2017 release of CiviCRM.  You can patch it now if you're patch-savvy.  Please try it and see if it solves your problem - or wait until January 2017, upgrade to CiviCRM 4.7.15+ and see if it solves your problem.