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