getI18nPaths() returns an associative list of domain => array of paths pairs, so the right way is:

$paths = $this->_arcavias->getI18nPaths();

if( isset( $paths['client/extjs'] ) )
{
    foreach( $paths['client/extjs'] as $list )
    {
        // scandir()
    }
}

if( isset( $paths['client/extjs/ext'] ) )
{
    // same as above
}