Some servers have limit on memory for PHP which leads to errors when you start adding new extensions. This guide will show you how to increase your PHP memory limit for Joomla! 2.5 using FileZilla. You can set memory limit to 32MB, 64MB or more.
This is how I fixed the error "Fatal error: Allowed memory size of xxx bytes exhausted (tried to allocate xxx bytes) in language.php on line 797" perfectly by increase the memory to 128MB.
Have fun!