How to translate a button? I have a website in English language and i want to translate some button in own language .
i know that i can add my language and simply translate all my stuff, but the point is i have to translate these buttons like subscribe in Simplenews module in en language.   

Here is my step :
i went to config/regional/translate/translate and add my word to translate but i could not do that .    

am i missing something ?
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.