_t() is a built in Dolphin function; you just call it on the page. You create the language key in the admin; settings -> language settings. and then you add in the text for each translation.
As an example, if you place the following in a php block; don't add the opening and closing tags in a php block, just the code:
echo _t('_my_about_text');
Then the output in the block will be your about text; you create the key _my_about_text in the admin and add in the text you want displayed. The function will pull out the correct translation based on the language selected by your members.
Of course that was a basic example. you would need to say a bit more about what you wanted to do exactly.