The i18n:extract task only handles and force XLIFF format by default, seeing this line in the sfI18nExtractTask.class.php file:
$extract = new sfI18nApplicationExtract(new sfI18N($this->configuration, new sfNoCache()), $arguments['culture']);
Here we can see that the sfI18N object is created without any source format in the option array (just the culture). The task should read the factories.yml configuration and extract from it several options to instanciate the object correctly, includinf the source format.
At least, the task should throw an exception if it is unable to extract strings from other formats than XLIFF files.
