We read Gmail in English because our browser support English and we send and receive messages in English, so how we can read Gmail in other languages? Answer is Google translation service.

You need to have Google toolbar in your browser. If your browser does not support Google toolbar you should install a browser that may support Google toolbar.
After installing it you can add a button to your toolbar. You can get that button here.
After adding that button do some simple tasks:
Sign in to Gmail
Click on settings on any page of Gmail which is on top right.
Select a language for you from drop down menu (that will be other than English)
Click save changes.

You can change your language of interface any time by visiting settings page.
Notes:
It translates menu and other links or navigation words from English to your chosen language but does not automatically translates your message in English, but you can do this by one click. And you can set it to do always.
Tip:
You can add more buttons in your Google toolbar by visiting this page.
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