How to translate a button?
You can use the module String Overrides which allow you to do what you asked for.

(As my low reputation I need to put here some more useless details :-) )

Copied From the module page:

Provides a quick and easy way to replace any text on the site.

Features:
Easily replace anything that's passed through t()
Locale support, allowing you to override strings in any language
Ability to import/export *.po files, for easy migration from the Locale module
Note that this is not a replacement to Locale as having thousands of overrides can cause more pain then benefit. Use this only if you need a few easy text changes.