The language id of the person browsing the site is always available within smarty templates through the following:

{$language_id}

While the software does allow you to set template sets specifically for each language you could also have a common template between both sets where you wish to include language specific text. In that case you could use the following:

{if $language_id == 1}
   This is English
{elseif $language_id == 3}
   this is Spanish
{/if}

You can find the language id for any language by looking within the following admin tool:

LANGUAGES > LANGUAGES HOME