Lines 53 and 55 of the Joomla libraries/joomla/installer/adapters/template.php relate to loading the language file. The specific lines are below. As you can see this is just relating to finding the name of the language file from the manifest
$this->manifest = $this->parent->getManifest();
$name = strtolower(JFilterInput::getInstance()->clean((string) $this->manifest->name, 'cmd'));
$client = (string) $this->manifest->attributes()->client;
I would suggest that line 53 is failing to find $this->parent->getManifest() and thus attributes is being called on a empty object (the $this->manifest part) and the instance can't be found of the $this->manifest again because it doesn't exist.
Read up on this Joomla Wiki Article if you don't know what a manifest is. I advise if you are permanently upgrading your template that you read through this and update your templates XML file to make sure it applies to all of this. As cppl says above as well also try reading through the upgrading a Joomla 1.5 template to 2.5 here. Specifically in your case the language section as this seems to be producing the error - however as the manifest is a very general thing through Joomla I would suspect this is just the first instance that is found - and would be systematic through the install!