how to : force a language for translated string

I am writing a piwik plugin (I use the unsupported existing one).

I want to log pages title, but only in one language (french).
Basically, it allows to avoid multiple titles to be tracked for the same page.

I found the way to force zenphoto items titles (`$_zp_current_album->getTitle('fr_FR')` as example).

But I didn't find the way to force translation of translated strings (as `gettext('Home')` or `gettext('Gallery')` as examples).

I tried the function `string dgettext ( string $domain , string $message )` used like that `dgettext('fr_FR', 'Home')`, but It does not not work.

Have you a solution to force translation? Or to well configure the function above?

thanks in advance for your help.